Chiefly British.
-adjective
1. one of a kind; unique.
2. done, made, or happening only once.
-noun
3.
a one night stand.
4. something done, made, or happening only once.
This began as a British expression but is now known in the US and
elsewhere.
Eddie:
Say what?! You base-jumped off
the Empire State Building? Were you insane?!
Johnny: Hey, it was only a one-off. I was
a turk lad at the time and a little drunk, and I don't think I'd ever have the courage to do it again.