A very informal way of saying that one heard a different story or explanation from the one just told. ("That ain't" is a colloquial way of saying "that isn't.") A: "Bob said he's retiring to spend more time with his family." B: "That ain't what I heard—word on the street is that he was caught embezzling company money." A: "She's just a friend, Janet! I just met her for a couple drinks and then went home!" B: "Yeah, that ain't what I heard, you no-good cheating bastard!"