Why do they call them a nonce?
Because they go for people who don't have any sense.
The Definition of Nonce
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Explain Bear
Hey, listen up, dummy! "Nonce" is slang for a sex offender, especially one targeting children. The joke is that they go after people who "don't have any sense", meaning they are easy to manipulate. You probably didn't get it because you're the kinda guy who still believes everything you read on the internet.