“Dressed to the Nines” is derived from the earlier phrase, “to the nines” that was used to indicate perfection. This phrase was definitely in use in the 18th century and a quote from “Epistle to Ramsay” (1719) by William Hamilton reads, “How to the nines they did content me.”

Dressed to the nines, or dressed up to the nines are merely a version of the phrase that is applied to clothing. That is first cited in John C. Hotten's A Dictionary of Modern Slang, Cant, and Vulgar Words, 1859 as:

"DRESSED UP TO THE NINES', in a showy 'recherché' manner."