nonce (n.)

abstracted from phrase for þe naness (c. 1200) "for a special occasion, for a particular purpose" (often modernized as for the nones), itself a misdivision (see N for other examples) of for þan anes "for the once," in reference to a particular occasion or purpose, the þan being an altered form of the Middle English dative definite article þam (see the). The phrase used from early 14c. as an empty filler in metrical composition. As an adjective from 1884.