-according to dictionary.com
1. harsh or bitter derision or irony.
2. a sharply ironical taunt; sneering or cutting remark.
-according to Urban Dictionary
1. A tongue of which the user speaks of something the complete opposite of what the user means. It often has the best comedic value.
2. Your body’s natural defense against stupid.
3. The bastard stepchild of irony.
4. The ability to insult idiots without them realizing it.
5. A word that gives you the possibility to play with your imagination when asked stupid questions.
6. Something that nobody on the internet can ever recognize.
7. A verbal reflex against stupidity. In my opinion one of the best types of humor.
I think I like the second group of definitions better! 🙂