As in, are you telling the truth??
Used by chavs etc.
A short way of asking a simple question -
warning, using it probably won't make you look cool, but quite the opposite -
people may just give you a wierd look and walk away.
Guy 2 - Woah! Yo fo sho mo fo?!
Guy 1 - What?!
Guy 2 - Are you fo sho - mo fo?! You're kidding?
Guy - Okay, so i'm going now...