In linguistics and psychology “nonce” words are fake words invented for a specific purpose (like to use as stimuli in an experiment). They have no meaning but should sound like plausible words for the language phonetically. In English some commonly used ones are “blicket” and “wug.” Ironically “wug” is so commonly used there’s actually a formalized “wug test” for morphological development.
I actually used to use this wrong, thinking “nonce” was a variation on “nonsense,” but it’s actually from the same origin as the cryptographic nonce: it’s a one-time-use word. So while they are often nonsensical (like basically all of the Jabberwocky poem) they can also be perfectly sensible and comprehensible, just with a one-time specific context of use.
(Also I’ve never heard nonce used as an insult of any kind. Is it a British thing?)