Any night that you have extensive sexual intercourse with no ejaculation. Named after the situation frequently found when you order a plumber: you stay in all day, and nobody comes.
Bob: Kelly came round last night.
Roger: I take it you left her looking like a Painter's Radio?
Bob: I couldn't get that far mate. Neither could she.
Roger: Ah, sounds like you had a Plumber Night.
Bob: You know all the cool sexual pseudonyms.