<"nonce" is prison cant for someone who is "doing time" for a sexual offence>

Rhubarb -- there is some discussion of this word in Marina Warner's latest book: No Go the Bogeyman: Scaring, lulling and making mock. (A fascinating study of ogres and giants, bogeyman and bugaboos which I finished reading just today.)

She says: The canting word for a sex offender in jail is 'nonce'. 'Nonce' means nothing; noncification turns someone into nothing.