British slang, used to describe sexual criminals
"...if it is found you have been sentenced for any sexual offence...you will be classed as a Rule 43 inmate or VP (vulnerable prisoner) and will be referred to as a "nonce" by other inmates."
- old Prisoner Information Leaflet
It's a scary word; something you'd never, ever want to be called in a serious way. The tabloids use it along with the more emotionally-charged "kiddy fiddler" to describe paedophiles and pedlars of child pornography, although traditionally, it has been used to describe the whole range of sexual criminals. From criminal and prison slang, it has made its way into our language through the media, who take enormous delight in using it to describe both suspected and convicted paedophiles.
Where it is used in an insult, it is usually taken very seriously. Call someone by this name, and you're likely to get a very, very hostile reaction indeed (although Bill Bailey has been described as "looking like a Klingon nonce" several times on the British TV show Never Mind the Buzzcocks without any apparent signs of violence).
Famous nonces include Gary Glitter, Jonathan King, Ian Brady and Chris Denning.
So where did it come from?
The origin of the word is uncertain, but several sources point to an acronym (actually a backronym) taken from Not On Normal Courtyard Exercise, referring to the practice in prisons of segregating those whose crimes are of a sexual nature ("Rule 43" as it is known). "Honour among thieves" seems to exclude exhibitionists, rapists and child molesters, and "nonce-beating" is a fairly common occurence in British (and presumably every state's) prisons, so they are separated from the rest of the prison population for their own protection..
It's quite possible that it has a London (maybe Cockney) origin. It is not to be confused with the word ponce (a pander or pimp), although the rhyming nature of it makes it tempting to form a connection, it has also been connected with nancy (oft-used to describe gay or effeminate men). In short, not a word for the polite dinner table.
Another possible origin lies in a word used to describe an envoy from the Pope - known as a nuncio (which the French call a "nonce"). It's not hard to make a connection with crimes of a sexual nature here, given the public view of priests in general (Catholic priests in particular), and the traditional Anglo-French distrust.
Polite use of "Nonce" - a dinner-table guide
The word is also used to describe either a nonsense word (as oft employed in Edward Lear's poetry), or one made up for a particular purpose, a one-off use, if you like. In this sense, it seems to have a Medieval origin, coming from a root anes, (meaning once). From "an anes" it likely became "a nanes" (in the same sense that the word "newt" followed from "an ewt"...)
Okay, I take it back. You may use it at my dinner table.
www.worldwidewords.org
http://fp.pmnwt.f9.co.uk/Prisonoffrinfoleaflet.htm
http://en.wiktionary.org/wiki/Nonce
LeoDV
randombit says re Nonce: How interesting! I bet this other usage makes cryptography conferences in the UK *very* odd. :)