dressed to the nines

O.E. nigen, from P.Gmc. *niwun (cf. O.Fris. niugun, O.N. niu, Du. negen, Ger. neun, Goth. niun "nine"), from PIE (e)newn (cf. Skt. nava, Avestan nava, Gk. ennea, Alb. nende, L. novem, Lith. devnyi, O.C.S. deveti (the Balto-Slavic forms by dissimilation of -n- to -d-), O.Ir. noin, Welsh naw). Ninth is O.E. nigonðe. Sense in cloud nine, etc., possibly because, "As the largest one-figure integer, nine is sometimes used for emphasis" [Shipley]. Nine to five "the average workday" is attested from 1959. Nine days has been proverbial since 14c. for the time which a wonder or novelty holds attention. The game of ninepins is attested from 1580.