In the UK, the term "nonce" is a slang word that primarily refers to a person who commits a sex crime, especially involving children, i.e., a pedophile. It is a highly derogatory term used mainly in prison slang and general British usage to describe sex offenders, particularly child molesters.
The term can also appear as "nonse" in some contexts or misspellings, carrying the same meaning. It is known to have a serious negative connotation and is used to stigmatize individuals accused or convicted of such offences.
Aside from this slang usage, "nonce" has entirely different meanings in other contexts, such as in linguistics where it means a word coined for a specific occasion, or in computing as a number used once for security purposes. However, in the UK slang context, it almost exclusively means a sexual offender against children.