Help wanted, UK! (or Ireland, or Commonwealth....)

Didn't come up in the archives, so it's vulgar? The word: nonce. From the Guardian's article on Maxine Carr: "'All the women were shouting at the windows. I had never heard the word "nonce" before in my life... I was being branded Myra Hindley mark two.'"

Nonce is UK prison slang for child molester. It has escaped into the community in recent years. I don't know the origin.