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
housePerson 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)!