I don't recall ever reading the term "one-off" before until Dick Cheney used it the other day while arguing that the Bush Administration had kept the United States safe, except for the Sept.11 attacks. Michael Quinion at World Wide Words explains it in detail; here's a piece of it:
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.)
If you're contemplating a sun-less Sunday and looking for things to do, check up the recently revivified Omniglot site with its fun listing of, shall we say, unusual phrases in different languages. Omniglot is a serious language site despite its fun moments, as is Ethnologue.
And The Engine Room has fun with "pre-loading." Hint: It has nothing to do with computers and software.