nine (n.) Look up nine at Dictionary.com
Old English nigen, from Proto-Germanic *niwun (cognates: 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" (cognates: 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.