One-Off

When I was a newly minted engineer I sometimes heard, and occasionally used, the phrase one-of to indicate an object was one of a kind. “That test set is a one-of.”

In recent years, I’ve seen one-of being replaced by one-off. I used to think one-off was a solecism, a corruption of one-of. But it’s not. A little research revealed one-off is a British expression that has crept into American English. Furthermore, one-off is likely the original and one-of is the eggcorn.

World Wide Words says this about one-off:

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”.)

So one-off means one of a kind, used especially to refer to a prototype. From manufacturing, the phrase spread into general usage, becoming applied to things that are not manufactured. “John is a character, a genuine one-off.”

To my non-British ears, one-of will always sound more natural – more one-of-a-kind-ish – and one-off will always sound a bit odd. And I’m okay with that.