Old English nigen, from Proto-Germanic *niwun (cf. Old Saxon nigun, Old Frisian niugun, Old Norse niu, Swedish nio, Middle Dutch neghen, Dutch negen, Old High German niun, German neun, Gothic niun "nine"), from PIE newn "nine" (cf. Sanskrit nava, Avestan nava, Greek ennea, Albanian nende, Latin novem (with change of -n- to -m- by analogy of septem, decem), Lithuanian devnyi, Old Church Slavonic deveti (the Balto-Slavic forms by dissimilation of -n- to -d-), Old Irish noin, Welsh naw).
Nine to five "the average workday" is attested from 1935. Nine days has been proverbial since 14c. for the time which a wonder or novelty holds attention.
