A “one-off” is something that is one of a kind or only happens once.
The phrase “one-off” originated in Britain in the 1930s. The Oxford English Dictionary defines the term “one-off” as an adjective (meaning “made or done as the only one of its kind; unique, not repeated”), and as a noun for such a product.
A one-off is 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 mold formed in sand.)
A modern 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.”
The phrase can be used for a special person, too, in place of the old, “After they made him, they broke the mold.”
We’re all one-offs, so we’re all special, right?