Old English nu "now, at present, immediately; now that," also used as an interjection and as an introductory word; common Germanic (Old Norse nu, Dutch nu, Old Frisian nu, German nun, Gothic nu "now"), from PIE *nu "now" (source also of Sanskrit and Avestan nu, Old Persian nuram, Hittite nuwa, Greek nu, nun, Latin nunc, Old Church Slavonic nyne, Lithuanian nū, Old Irish nu-). Perhaps originally "newly, recently," and related to the root of new.
Often merely emphatic; non-temporal usage (as in Now, then) was in Old English. The adjective meaning "up to date" first recorded 1967, but the word was used also as an adjective in Middle English with the sense "current" from late 14c. Now and then "occasionally" is from 1530s; now or never attested from 1550s.