Until this year I had not heard the expression "one-off". When I first heard it, I thought that someone had mistakenly written this instead of "one-of", as in "one of a kind". But now I even have heard it on TV, so it is not a typo. According the online Merriam-Webster Dictionary, it has been around since 1934.
Is it principally a U.K. or Canadian expression?
As opposed to American, that is? I'm American and find nothing unusual in the expression, though we do tend to use the formulation one-shot+ more frequently than +one-off.
I'm American too. The first time I heard "one-off" I heard it from a British University. It was in a context that was too formal for "one-shot" so I think the expression is less slangy than "one-shot" is. I think it's worked its way over to the US now, or I'm just used to it from having lived in the UK.

I always find it interesting that you (ie me) can presume that an expression in your own variety of English is widely-used throughout the English-speaking world, when in fact, it isn't.
The question is, "One off of WHAT?" I guess my mistake is to expect that language is logical.
People speak the way they want to, and it is up to us to try to understand what they mean if we care about them.
Thanks for replying to my question.

"Once off" is also quite common (sometimes with a hyphen): a once-off payment, a once-off adjustment.
I've never thought to wonder "off of what", I think it's one of those phrases that just has to be considered as a whole.
Edited by: alan1972
This American would be more likely to use "one time." A one-time payment, a one-time adjustment.
Michael Quinion says
> 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.”
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