1. one of a kind; unique.
2. done, made, or happening only once.
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.
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.
Damn, you really had a one off with that hot soccer MILF behind the beer stand?