Used to address someone's incorrect assumption of you. (Usually followed by a qualifying statement like "because I don't care" or "... that would be fooled by that" etc)

Person A: If you give me your phone number, I'll buy you a house

Person B: YMMFTT!

Or

Person x: I know we broke up a year ago, but my current relationship isn't going so well.

Person y: YMMFTT bc idc (because I don't care)!

phone: why don’t you go to sleep tommy it’s 2 in the morning?
tommy:why don’t you die you dumbazz