U.P. is a reference the great Joycean work, Ulysses. Dennis Breen receives a postcard with just those letters. It excites his madness. Perhaps it is a reference to in ability to satisfy his wife in husbandly manner. Perhaps it’s a reference to You urinate, You’re no good, You put your fingers up your anus, and You can’t get it up any more. Or simply, a reference to Ulysses and Penelope. IOU.