"dressed to the nines"

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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.