nonce /nɒns $ nɑːns/ BrE AmE adjective [only before noun] technical
[ Date: 1100-1200 ; Origin: then anes 'the one purpose' , mistaken for the nanes ; anes from an 'one' ( ⇨ ↑ one 3 ) ]
a nonce word or phrase has been invented for a particular occasion and is only used once