Dressed to the nines

This expression, meaning "very fashionably and elaborately dressed", is recorded from the 18th century.  "The nine" or "the nines" were used to signify "superlative" in numerous other contexts.  Theories include:  9, being the highest single-digit number, symbolized the best; a metanalysis of Old English to then eyne "to the eyes"; and a reference to the 9 muses.

See http://www.alt-usage-english.org/big_faq.html#fxjerryb