A form of the question "Why the hell" or "Y the hell." Similar to WTH (what the hell) but using a different W word that can't be expressed as easily with a WTH.
Guy: Yo man today I had lunch with that janitory lady at school.
Guy: I know right lolol
Fred: no, WHY the hell?? idiot
Guy: Oh, stupid, write yth next time and maybe I'll KNOW WHAT YOU'RE SAYING
why the hell
yth did u do that!?!?!?