There is yet another meaning for "nonce" - unfortunately. I almost feel that I should not bring it in to this thread, especially knowing Jackie's predilection for swans - and I agree that Helen's definition is far pleasanter than mine.

But in the interests of completeness, to produce as near as possible a full rotundity of nuance and meaning, I have to tell you that "nonce" is prison cant for someone who is "doing time" for a sexual offence. They are people who are universally abhorred by the rest of the inhabitants of the prison - staff and cons alike. They are frequently assaulted and very often have to be segregated for their own safety. The worst cases are removed to separate wings of the jail, or even to separate jails.

I have no idea at all where or why the term acquired that meaning.

The term has spilled out of jail, and I have occasionally heard it used - in the same context - in civilian life.