But first, some trivia! "Euphemism" was first recorded in its sense of using a less offensive or distasteful word in place of the one actually meant in 1793. It comes from the Greek euphemismos, or "use of a favorable word in place of an inauspicious one," and from euphemizein, "speak with fair words," and originally had to do with avoiding ill-omened words during religious ceremonies. Today we mostly use euphemisms as more polite or socially acceptable words or phrases in place of very impolite ones, and those are the ones we're interested in for the purposes of this article.
Covered thoroughly and eloquently here by aqua_crescent, shit is a very versatile expletive. So it makes sense that the euphemisms for it would be as numerous. These words include, but are not limited to, shoot, Shinola, sugar, shucks, crap, crud, kaka or caca, doodie, puckey, shite, shiznit, and jack, a shortened form of "jack shit." They can be used in any of the ways that shit can, but some are more commonly used in one or two ways than in others.
For example, shoot, shucks, and sugar serve best in an exclamatory fashion:
Dean rummaged all the way to the bottom of the duffel bag. "Dude. Where's the EMF detector? Hey, where's the rock salt?"
Sam blinked. "Ah, shoot! I think I left it on Bobby's workbench."
It was one of Ralphie's favorite daydreams. He'd rescue all the womenfolk from Black Bart, and then the adoration would begin. "Why, thank you, Sheriff," the rancher's pretty daughter would simper.
But the Sheriff was a loner. His spurs would jingle as he stepped into the street. "Shucks, ma'am. It weren't nothin'," he'd say with a tip of his hat. And then he'd swagger off into the sunset.
Crap, crud, and others can also be used as general exclamations, but they function as simple replacements for the general meaning of shit, too:
"Cut the crap, Crowley," Bobby said. "The contract's airtight. You can't send me to Hell today."
However, please note that crap and even crud, while not usually considered curse words, are still impolite in many places. They're more offensive than shoot or sugar, for instance, and the Winchester boys and their friends (and frenemies) would be far more likely to use them than someone like Ralphie or the Leave it to Beaver gang.
Jack, along with crap, crud, and shiznit, stands in for shit in the sense of knowledge or general stuff. Puckey (or even hockey) serves as a short version of "bull puckey/hockey" or "horse puckey/hockey," which stands in for "bullshit":
"So, Cas, do you remember—" Sam began.
"Shut up, Sam. He doesn't remember jack," Dean sighed. "Nobody does when they first come back from Hell, even fallen angels."
"Horse puckey," Bobby snorted. "He was an angel. Different rules."
Most of these euphemisms are fairly modern, though, and some are more common in certain areas—sugar, for instance, is almost exclusively used in the U.S. South—so make sure you check your source material to see if they're appropriate for your characters and setting.
lady_ganesh filled us in on fuck over here. While the f-word packs quite a punch, some of its much milder substitutes run the gamut from the silly to the sweet, like fiddlesticks, screw, futz, fart, eff(ing), flip(ing), freak(ing), frig(ging), frick(ing), bugger(ing), and fudge.
Words like fiddlesticks and fudge are innocuous enough to use as exclamations in more "polite" fandoms:
The pie slid from the plate to the floor, and the Cleaver boys stared in dismay and horror.
"Oh, fiddlesticks," June muttered, looking uncharacteristically flustered. "Boys, I told you to pick up your baseball things! Now look what's happened. There'll be no dessert for you tonight."
The lugnuts went flying from Ralphie's hand. "Ohhhh, fuuuuuuuudge!" he said.
Only he didn't say fudge. He said THE word, the big one, the queen-mother of dirty words, the F-dash-dash-dash word.
The rest of the words, though, especially screw and bugger, are far less polite:
"Oh my God, screw you, Crowley," Dean shouted. "We're not taking the deal. Give us Bobby's soul!"
"Screw me? Oh, please. Bugger you, Winchester," Crowley snarled, eyes narrowed. "It was a fair deal."
Please note that screw is much more likely to be used by American English speakers, and bugger, a vulgar term for sodomy, is most likely to be used by British or global English speakers.
The rest of the euphemisms work well in an adjectival or adverbial fashion, standing in for fucking:
The dumbasses of Burkitsville, Indiana, "Home of Indiana's Best Apples!!!", were sacrificing tourists to a bloodthirsty harvest god. Awesome. "I hope your apple pie is freakin' worth it," Dean said in disgust.
It was three a.m., Sam was still working on the broken EMF detector, and Dean was strongly contemplating fratricide.
"Would you quit screwing around with that effing thing? Some of us are tryin' to get some friggin' sleep, here!"
Again, most of these euphemisms are more appropriate for fandoms with modern settings. But that's not to say you couldn't have characters from today get dropped in the past and bring their fancy euphemisms along.
Religious-themed euphemisms are numerous and popular, probably because blasphemy was far more offensive than vulgarity in English for hundreds of years. Some of the more interesting blasphemous curses, many of which have morphed and are now considered euphemistic, are featured in randi2204's article on Shakespearean curses. Several of these euphemisms survive today, along with newer ones: darn, dang, dad gum, doggone, drat, gosh, golly, gol-durned, tarnation, gum, jeez or gee whiz, jeepers, Jiminy Cricket, jinkies, Jebus, crikey, criminey, cripes, heck, H-E-double-hockey-sticks, blazes, Sam Hill, and dickens.
The blasphemous euphemisms above can be broken down into a few general groups. One group stands in for damn, generally considered in itself to be one of the milder curse words, and can be used in the same ways:
Wally looked at the glue-covered ruin of the model plane in dismay. "Aw, darnit, Beav. If you'd just asked, I woulda helped you with it!"
"Hey, Dean? Where'd you put the dang holy water? It's not in the Impala," Sam called.
Although words like hell and devil aren't considered to be particularly offensive or strong today, they were much more offensive in the past and still remain very impolite in some places.
Ward found Beaver's bike propped up against the fence, covered in dark creek mud. What in the Sam Hill had he been up to? He was generally a good boy, but sometimes Ward was afraid he'd go to the dickens.
Another group consists of euphemisms for God:
Eddie Haskell's eyes were wide and innocent. "Gosh, Wally. I didn't know you still needed that baseball glove. I thought you'd want it to go to a good cause or something."
"We don't need your gol-durned help," the deputy said to Dean. Dean tried not to laugh. It seemed people really did talk like bad Westerns in 1880s Nevada.
Still another group contains euphemisms for Jesus:
"Oh, Chri- I mean, cripes." Dean corrected himself hurriedly at the look on Castiel's face. Angels got cranky when people blasphemed.
"Jeez, Dean," Sam said after Castiel left. "You could've been a bit nicer."
"'Jeez'? What are we, Wally and the Beav?" Dean put on an earnest, wide-eyed look and pitched his voice higher. "Gee whiz, I'd hate to offend an angel by saying 'Jesus'! Except," he said, voice returning to normal, "angels are dicks. Even Cas."
As with all the other euphemisms, there are appropriate and inappropriate places for these. After all, Dean Winchester would think nothing of saying "Jesus" or "Christ," but in the presence of an angel with the ability to smite when offended, he reverts to the euphemisms we often use in childhood. Depending on when and where you (or your characters) grew up, that'll change the euphemisms you pick when it's inappropriate to resort to outright blasphemy.
And of course there's bleep, which is the universal euphemism in English, thanks to the charming sound we hear when a censoring body feels the need to drown out an offensive word spoken during a radio or television broadcast. It can be used in pretty much any way you want. Since it's universal, it's also appropriately versatile.
"What the bleep was that?" Sam asked.
"Wait, did you just say 'bleep'?" Come on, who even says that! Are you bleeping me?" Dean froze, a look of horror on his face. "Bleep! What the bleeping bleep is this?"
Castiel tilted his head as though he were listening to something. "Gabriel. I believe my brother has resumed his Trickster ways."
Sam and Dean exchanged resigned glances as their clothes were replaced by scrubs and theme music from Dr. Sexy, M.D. began to swell around them. Well, that was just bleeping great.
Not every broadcasting body uses bleep, though. Just as an example, some cobble together ridiculous phrases, such as melon farmer, monkey-fighter, or even Monday-to-Friday for motherfucker, in order to avoid offensive language and make a program "safe" for family viewing. Of course, the replacements aren't limited to the eff-word. You can find a bunch of these hilarious substitutions over on TV Tropes if you're interested.
So when should you use euphemisms? Whenever your characters, fandom, setting, audience, style, or rating call for a bit of rough language that's not too terribly rough, reach for a euphemism. Just make sure you check to see that they're appropriate, time- and geography-wise, for what you're writing. And whatever you write, have a darned good time doing it!
Thanks to chomiji and green_grrl for many of the awesome links and euphemisms!
"Bowdlerise: Film" at TVTropes.org
"Euphemisms" at Dictionary.com
"Mild Swear Words" at Everything.com
"Quotes from A Christmas Story" at IMDB
"Quote of the Day" at The CW Source
"Sam Hill" at Wikipedia
"Shiznit" at StraightDope.com
"Southernisms" at YourDictionary.com
"Swear Words, Taboo Words, and Euphemisms"