A person of thoroughly bad character.
Example:
Finally, on the verge of physical and financial ruin, the reprobate dropped his lowlife friends, joined AA, and begged his wife to come back.
Reprobate (which includes the prefix re-, "back or backward") is often said in a tone of joshing affection. The related verb is reprove or "scold," since the reprobate deserves a constant scolding. Shakespeare's great character Falstaff -- a lazy, lying, boastful, sponging drunkard -- is the model of an old reprobate.