If someone had asked me I'd said it's derogatory, like "you fucking nonce!", but also it's a religious thing (not specifically pedo related 😁) but all I could find was "slang for pedo".

So I doubled down and here is where I went wrong:

The Nonce apostolique is french for Nuncio in English.

So nonce is pedo in English slang, and in french it's a diplomatic representative of the pope.

How appropriate!

Cheers