Guy1: Your mother was fun in bed last night
Guy2: What?
Guy1: What?
Santa Claus: What?
Satan: What?
God: What?
Your mother.: What?
Everyone else: What?
literally "Your mother". A derivation of yo mamma, used as a jokeinsult for any occasion. It doesn't have to make any sense at all and can be part of a phrase. Often used in a sexual context.