work (one's) tail off
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 tail off at the gym! Sally and Jim work their tails off when we hold our user conference—I don't know what I'd do without them!
work one's tail off
and work one's buns off; work one's ass off; work one's butt offInf. Fig. to work very hard. (Use caution with butt.) I worked my tail off to get done on time. You spend half your life working your butt off—and for what?