Young Buck -- Urine Big Trouble Now

Young Buck could go to prison for faking a piss.

The rapper -- who is on a probation -- used someone else's urine to pass a drug test ... so claims his probation officer.

The P.O. says Buck came in for his drug test and concealed a plastic bag with someone else's clean pee, and then used a tube to transfer it into a sample cup.

Turns out it's a felony to falsify a court-ordered urine test, punishable by up to 5 years in prison.