Inappropriate:

There are only a few appropriate answers to any question. If you want to be funny, respond with an answer that is inappropriate in some way. For instance, consider an IM asking what you are doing. You might reply:

1. Dancing with a friend => Completely appropriate, humor level zero
2. Dancing with a lover =>  Mildly risque, humor level plus one
3. Dancing with a foe    =>  Inappropriate, nobody dances with the enemy, humor level plus four.

Of course, you have to be selective with the inappropriateness of your response. For example, you could reply to the question above with, "what's it to you?" That's not funny, that's hostile.

Hostility is almost always appropriate and therefore not usually humorous.