The first or second in an almost ritualistic chain of insults. Can also be used as a stand alone insult. It is best used by mixing it with part of the first speakers sentence. Often used when A: it makes no sense, or, B: it makes way too much sense.
Person 1: That's one tall building
Person 2: Your MOM is one tall building!
Person 1: Your FACE is one tall building!!
Person 2: Your MOM'S FACE is one tall buidling!!!
Everyone else in the room: Ooohhhh!!
Person 1: I don't think i'm gonna to make it to friday.
Person 2: Come ON man!
Person 1: Hehehe... I'll come on.... YOUR MOM! OH!!