1.) Slang term for a person who has committed crimes of a sexual nature, particularly pedophilia, esp. in the United Kingdom. Comes from the acronym used in prisons to describe said individuals: Not On Normal Communal Exercise. Also used occasionally as a general insult, regardless of the tendencies of the person to whom the word is applied.
2). Also, somewhat obscurely, a term meaning 'the present or particular occasion.'
1). 'Ere, this nonce pedobear is in for right ear-boxing if 'e don't stay away from the main prison population.
Ay, mate, what say you lend a chap 5 quid, eh? No? Oh, what a right nonce you are!
2). I say old chap, it appears that I have been incarcerated because pedophilia is rather illegal for the nonce. Fortunately for sexual deviants, the legislation is becoming more and more lenient as time goes by in good old Great Britain.