In UK slang, the term "nonce" is a derogatory term primarily used to describe someone who is a sex offender, especially one who has committed crimes involving children, such as a pedophile. It is used in a very negative sense and can extend to referring to someone accused or convicted of such offenses. The term carries significant social stigma and weight in its use. This slang usage is particularly prevalent in British criminal and prison contexts.

Besides this slang use, "nonce" also has other meanings, such as a cryptographic term or a nonce word in linguistics, but in UK slang, it specifically refers to the derogatory term linked with sexual offenses against children.

Thus, in the UK, calling someone a "nonce" is a serious insult implying they are a sexual offender, particularly a child molester.