Stands for "This Is Really Funny". You use it when you find something really funny. Used by people who are sick of hearing and using lol.
Man: Oh man tirf!
Man: Oh man that's tirf!
Abbreviation for That Is Really Funny (it's the new "lol"). It can also be used as an adjective to describe something funny, and when people ask what it means, you tell them, and then they say it doesn't make sense, and then you say to yourself: tirf.
That comment you made about that whore is really tirf.