[This is a fast-access FAQ excerpt.]
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.