From Longman Dictionary of Contemporary Englishwork/play etc your butt offwork/play etc your butt offAmerican English spoken to work, play etc very hard I worked my butt off in college. → butt
Examples from the Corpus
work/play etc your butt off• Meanwhile, Inspiral Carpets went in at grass roots level and worked their butts off in the clubs.• I had to give the ball up, and then I had work my butt off to get it back.• You could have worked your butt off helping a rep and you finally got the rep doing everything right.• He took a beating today but he played his butt off.• In short, I worked my butt off.• I work my butt off for you, while that restaurant is doing worse and worse.• I worked my butt off in basketball and stayed on the varsity-in fact, did well. 