Stands for: That's What The Fuck I Said.
Used when someone repeats something you said or when you are agreeing with someone who shares the same opinion as you.
Girl 1: And I was like, ' It's obviously seven!!"
Girl 2: No you didnt. I answered that question.. TWTFIS!
Boy 1: She's so dirty, why would he put his penis in that?
Boy 2: I know! TWTFIS