The story
From one + off. Probably from foundry work, in which molds are expensive and generally made to be reusable. A term long used by artists who are printmakers and sculptors to indicate a unique print or casting. If using traditional edition numbering, a one-off would be a "1/1", which is said, "one of one" meaning the first print in an edition of one.
Etymology from Wiktionary, CC BY-SA 4.0.