I was pretty firmly in the camp of whoa when I got this question. My New Oxford American Dictionary doesn't list woah at all, and under whoa it says:
used as a command to a horse to make it stop or slow downAnd the Corpus of Historical American English—which tracks language usage through magazines, newspapers, and fiction and nonfiction books—lists 911 uses of whoa in published sources versus ten of woah.
[informal] used as a greeting, to express surprise or interest, or to command attention
Seems pretty conclusive, right? Well, it wouldn't be a post if that was the end of the story. Let's look at the evolution of uses of whoa via examples from Warner Brothers cartoons, Pulp Fiction, Iron Man 2, The Incredibles, Bill and Ted's Excellent Adventure, and Finding Nemo.
The most literal use of whoa is to stop or slow a horse, or in the following case, a camel.
Yosemite Sam: When ah say, "Whoa!" ah mean "Whoa!"(Note to writers who have serious riders as characters: Most horses are actually trained to stop at the command "Ho!")
Of course whoa can also be used figuratively, in any case where you want someone to slow down or stop.
Vincent: Lance! It's Vincent. I'm in big fuckin' trouble, man. I'm coming to your house.From there, whoa can involve any situation in which the speaker wants the conversation to slow or stop so that the unbelievableness of what is happening can be taken in.
Lance: Whoa. Whoa. Hold your horses, man. What's the problem?
Nick Fury: Whoa, whoa, whoa. He took it? You're Iron Man and he just took it? The little brother walked in there, kicked your ass and took your suit? Is that possible?And that is a close cousin to a general "Wow!"
—Iron Man 2
Helen: He put a tack on the teacher's chair. *During* class.And the film that made whoa famous as an all-purpose stoner/surfer/Southern California-ism:
Dash: Nobody saw me. You could barely see it on the tape.
Bob: They caught you on tape and you still got away with it? Whoa! You must have been booking! How fast do you think you were going?
Ted1: OK wait. If you guys are really us, what number are we thinking of?Bill and Ted types are delightfully parodied by the sleepy-voiced turtle in Finding Nemo:
Bill2, Ted2: 69, dudes.
Bill1, Ted1: Whoa.
—Bill and Ted's Excellent Adventure
Crush: Saw the whole thing, dude. First you were all like "whoa", and we were like "whoa", and you were like "whoa…."
The rise of the alternate spelling woah seems to go along with its use in the "wow" sense, probably helped along by Keanu Reeves' pronunciation of the word, which sounds more "woe" than "whoa." (It has also increased because of the greater number of people posting on the internet without copyeditors to correct their spelling.) But as far as American dictionaries are concerned, whoa is the only official version. Using woah would be something to insert sparingly into fiction dialogue or tight POV narrative, if you were intentionally making a statement about a Bill/Ted/Crush-like character's accent.
But it turns out there's a British English versus American English aspect to this question, too.
Merriam-Webster's word.com newsletter seems to indicate that the usage of woah is growing in a direction that might someday lead to it being recognized as a legitimate variant. A linked article cites the Oxford English Dictionary's history of whoa as:
who c.1450-1859So woah does have a legitimate history as an anachronistic variant of a variant. (Although whoa, oddly, also appears to be anachronistic, as noted by the article author.) More contemporarily, the author notes increased sightings of woah in British advertisements, newspapers, and websites. So despite the Keanu effect, it may be British dictionaries that recognize woah first!
woah 1856 (one example—included under the headword woa)
whoa 1843-1898 (but, of course, we know it's still used)
What is a writer to do? The dictionaries haven't changed yet.
- If you are writing about horses and riders, or other literal or closely figurative "stop!" situations, use whoa.
- If you are writing nonfiction, or a character with a formal voice, use whoa.
- If you are writing a "wow" situation and want to portray an American stoner or surfer type of character or an informal British character, you might consider using woah.