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In the United Kingdom, nonce is a slang word for a sex offender or child sexual abuser. Although the term traditionally referred specifically to sex offenders, it later became a more general term of abuse approximately synonymous with "idiot". 
Usage in English prisons
Nonce first came into widespread use in UK prisons, where it is primarily used by prisoners to refer to convicted sex offenders, especially abusers of children. "Nonces" are traditionally targets of physical abuse from their prison inmates, and so usually go on Rule 45 (formerly Rule 43), the rule that enables the segregation of vulnerable prisoners from the other prisoners for their own safety as the prisoners were originally designated N.O.N.C.E.('Not On Normal Courtyard Exercise'). The Rule 45 section of British prisons in which sex offenders are segregated (also known as going on 'The Numbers' or, in rhyming slang, 'The Cucumbers') is often referred to as the "nonce wing".
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The Oxford English Dictionary, while describing the word's etymology as "Origin unknown", states that it is "perhaps related to" nance and cites a quotation which claims the word was derived from nancy-boy (a derogatory term referring to effeminate or homosexual males). It also suggests that it may derive from the nonse, Lincolnshire dialect for "good-for-nothing fellow".