papain n a proteolytic enzyme obtained from the unripe papaya; used as a meat tenderizer Usage(s) If papain had the same effect on human slipped discs, he reasoned, they would shrink back into place. One evening, he gave adolescent rabbits intravenous injections of papain. If children cried for Castoria in those days, they kicked papain the shins for Buster Brown and his sweetheart, Mary Jane, and his dog, Tige.