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flat on (one's) ass
rude slang On one's buttocks. Usually used to describe one's resting position after a fall. Oh, those steps were a sheet of ice—I fell flat on my ass!
flat on one's ass
1. Fig. Inf. completely exhausted. (Potentially offensive. Use only with discretion.) I'm just flat on my ass. I need some rest. After the day of the marathon, Pete was flat on his ass for a week.
2. Fig. Inf. broke; financially destroyed. (Potentially offensive. Use only with discretion. An elaboration of flat broke.) Sorry, I can't help you. I'm broke—flat on my ass. The guy's flat on his ass. Can you help him out with a loan?