abstracted from phrase for þe naness (c.1200) "for a special occasion, for a particular purpose," 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 from 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.