A legendary figure, who's the subject of a recurring joke. The objective of this joke is to make someone ask "who's mushtaq". But they must already know about this joke from previous experience, and are tricked into asking it all the same. Then you hit them with the above given statement.
This is ideally done in a group setting.
Guy # 2: Wait, who's Mushtaq
... long pause, which makes Guy # 2 realize he's fucked up. and then he gets hit with it:
"the guy who fucks your mom"