Pejorative term for a sexually deviant male. Current usage has shaded into implying a pedophile but the older meaning was a homosexual or effeminate male. The transition in meaning has occurred as homosexuality has become more acceptable but pedophilia has become less so.
Etymology: Nance, from Nancy. The female name applied derisively to a male expressed the speakers opinion of his taking or deserving a female or inferior role. Also Nancy-boy, Miss Nancy, Miss Molly, Nauncy, Naunce, Nanny-boy.
Variants of this term date back to at least the mid-nineteenth century and probably much earlier.
The charge was elevenpence, but it was a dark, evil-smelling place, and a notorious haunt of the 'nancy-boys.' Downstairs, in the murky kitchen, three ambiguous-looking youths in smartish blue suits were sitting on a bench apart, ignored by the other lodgers." George Orwell, "Down and Out in Paris
"...My first presumption was that he wished to ask me my direction but as soon as he spoke I knew him to be a nonce..." John Stewart Earl, Journal