nonce

A nonce is something created or used for one particular occasion only.

A nonce word is coined for a single use, not meant to enter the language permanently—like Joyce's "scrotumtightening" or a parent calling a child's blanket a "blankie-wankie." The poet invented nonces freely.

That's a nonce formation, not a real word.

Separately, in British prison slang, nonce is a serious term for a child molester or pedophile—prisoners consider nonces the lowest of the low, subject to violence from other inmates.

The slang has spread to internet usage as a harsh accusation: He's a nonce who belongs in jail.

The connection between the two meanings is unclear.