Originally in reference to the 1966 art-heist film How to Steal a Million, starring Audrey Hepburn and Peter O'Toole.
A lostpainting is epitomized by this quote from the film:
Charles Bonnet (O'Toole): What have I done? I've given the world a precious opportunity of studying and viewing the Cellini Venus.
Nicole Bonnet (Hepburn): Which is not by Cellini!
Charles Bonnet: Ahh, labels, labels. It's working with the Americans that's given you this obsession with labels and brand names. I wish you'd give up that ridiculous job.
Homosexual 2: I assure you, he's merely a lostpainting.
Homosexual 1: *Frowns sternly at the aforementioned passer-by*