What’s up with that? Definition 1 of 2Pro Tip 1/3Informal Usage Used in casual situations among friends or peers, not in formal contexts. You're wearing flip-flops in winter, what's up with that? Pro Tip 2/3Expressing Annoyance Can indicate irritation or annoyance about the situation in question. He left without cleaning up, what's up with that? Pro Tip 3/3Seeking Explanation Used to prompt an explanation for something odd or unexpected. This app just crashed again, what's up with that?