work your ass off meaning: to work very hard. Learn more.
(idiomatic, slang) To cause a person to work excessively or to the point of exhaustion. (to work excessively): work someone's butt off, work someone's tail off.
rude slang To work really hard (on or at something). I know you thought it was just a silly New Year's resolution, but I've been working my ass off at the gym!