Let's take a look with the assistance of our friends at NCIS.
It's easy to confuse loath and loathe, especially given that both the meaning and etymology of the two words is similar – loath (sometimes spelled "loth") coming from the Old English lāth meaning ‘hostile, spiteful', and loathe from the Old English lāthian. What's more obvious in the Old English is the difference between the two words, since in modern English we have a smaller difference in spelling but a more significant difference in meaning.
Loath is an adjective meaning "reluctant, unwilling", so if you are "loath to do something", it means that you really don't want to do it. For example:
Ziva was loath to give her gun to the guard, even though the security protocols required that she do so.As you can see from these examples, loath can be used as a synonym for any word meaning reluctant, though it's probably less likely to be seen in common usage. In addition, possibly because loathe implies an intensity of emotion as we'll discuss shortly, loath seems more likely to be used to describe a strong reluctance or unwillingness than a simple reluctance or unwillingness. You may want to keep that in mind when using loath in your writing.
Gibbs was loath to believe that the similarity between the two cases was a coincidence.
As we implied in the last paragraph, loathe is a verb meaning to "feel intense dislike or disgust for". Some examples of using loathe are:
Kate shuddered at the thought of having to interact with Ari again -- she had loathed the man on sight.So if you're going to be showing your dislike, the verb loathe is what you should be using. But if you're somewhat reluctant about showing your dislike, then you're loath to express that emotion. How do you remember the difference between the two? If you loathe something, then you hate it -- and both words end in an "e". If you are loath to do something, then you're unwilling to do so, and there is no "e" in either word.
Tony turned away from where Ziva and Michael were talking quietly, trying not to show his loathing for the man.
Loath definition: http://oxforddictionaries.com/definition/english/loath?q=loath
Loathe definition: http://oxforddictionaries.com/definition/english/loathe?q=loathe