I would check your source, Daniel. The sentence does not sound right.
I would accept "You got to work off your body fat" or something similar, but not "work your body off".
The idiom has at least one other meaning not mentioned by Eduardo. "To work off" can mean to pay for something with work. Let's say that you borrow $100 from me. You tell me later that you simply cannot repay the loan. I ask you if you want to work it off. You agree to work 10 hours for me, "working off the loan" at the rate of ten dollars per hour.