Coming from the ever so famous word "douche," a douche pup is one who is a douche in the making. A semi- douche. A bastard.
Person A: "Hey baby! Want to see my rather large shennanigan?"
Woman: "You bastard! How dare you!"
She walks away
Person A: That line never works! I don't get it! Its huge!
Person B: "Hey listen here ya douche pup! Ladies prefer gentlemen!"