Q From Jim Brewster: Whence the term one-off, which obviously means ‘one of a kind’?
A This began as a British expression but is now widely known in the US and elsewhere, I am told.
It comes out of manufacturing, in which off has long been used to mark a number of items to be produced of one kind: 20-off, 500-off. This seems to have begun in foundry work, or a similar trade, in which items were cast off a mould or from a pattern (“We’ll have 20 off that pattern and 500 off that other one”.) An example is in a book of 1947 by James Crowther and Richard Whiddington, Science at War: “Manufacturers found it very difficult to give up mass production, in order to make the 200 or so sets ‘off’.”
A one-off was just a single item, used in particular to refer to a prototype. The first known example appeared in the Proceedings of the Institute of British Foundrymen in 1934: “A splendid one-off pattern can be swept up in very little time.” (The reference is to a casting mould formed in sand.)
Out of this came our current figurative sense of something that is done, made, or happens only once — as you say, one of a kind. An example appeared in the Coventry Evening Telegraph in February 2006: “Anyone who would like to donate in Mo’s memory is welcome to make a one-off donation or more long-term contributions.”
It can also be used of a special person, someone for whom it might be said “After they made him, they broke the mould”. Here’s an example from the Daily Telegraph of 13 April 2006, about Michael Eavis, who runs the Glastonbury Festival: “I have great respect for him. He’s a fantastic eccentric, really, a one-off.”