Dressed to the nines
Dressed flamboyantly or smartly.
Nine is the most troublesome number in etymology. There are several phrases of uncertain parentage that include the word. Examples are, cloud nine, nine days' wonder and the infamous whole nine yards. We can add 'dressed to the nines' to that list.
The most frequently heard attempts to explain the phrase's derivation involve associating the number nine with clothing in some way. One theory has it that tailors used nine yards of material to make a suit (or, according to some authors, a shirt). The more material you had the more kudos you accrued, although nine yards seems generous even for a fop. Another commonly repeated explanation comes from the exquisitely smart uniforms of the 99th (Lanarkshire) Regiment of Foot, which was raised in 1824. The problem with these explanations is that they come with no evidence to support them, apart from a reference to the number nine (or 99, which seems to be stretching the cloth rather thinly). The regiment was in business in the early 19th century, which is at least the right sort of date for a phrase that became widely used in the middle of that century.
The first example of the use of the phrase that I can find in print is in Samuel Fallows' The Progressive Dictionary of the English Language, 1835. In his entry for the phrase 'to the nines' Fallows gives the example 'dressed up to the nines' and suggests that it "may perhaps" be derived from 'to thine eynes' - to the eyes. Not bad as a hypothesis, but without any evidence (and I can find none) 'may perhaps' is as far as we can go with that.
What counts against the above explanations, and indeed against any of the supposed explanations that attempt to link the number nine to some property of clothing, is the prior use of the shorter phrase 'to the nine' or 'to the nines', which was used to indicate perfection, the highest standards. That was in use in the 18th century, well before 'dressed to the nines' was first used, as in this example from William Hamilton's Epistle to Ramsay, 1719:
The bonny Lines therein thou sent me,
How to the nines they did content me.
It is worth noting that the number nine has long been used as a superlative. The Nine Worthies were characters drawn from the Pagan and Jewish history and from the Bible. This distinguished group consisted of Hector, Alexander, Julius Caesar, Joshua, David, Judas Maccabaeus, King Arthur, Charlemagne, and Godfrey of Bouillon. These were well-known to mediaeval scholars as the personification of all that was noble and heroic. Also, classical mythology has given us the Nine Muses of Arts and Learning - Clio, Thalia, Erato, Euterpe, Polyhymnia, Calliope, Terpsichore, Urania and Melpomene.
The Poetick Miscellenies of Mr John Rawlett, 1687, provides the earliest reference to 'to the Nine' that I can find:
The learned tribe whose works the World do bless,
Finish those works in some recess;
Both the Philosopher and Divine,
And Poets most who still make their address
In private to the Nine.
It seems clear that 'the Nine' that Rawlett was referring to were the Nine Muses. It is just as clear that 'dressed to the nines' is merely an extension of 'to the nine/s' and that we could equally well 'dance to the nines' or 'etymologise to the nines'. The search for the link between 'nines' and dress sense has unearthed no convincing candidates. Absence of evidence isn't evidence of absence, but I'll stick my neck out here and say, with this phrase and with the other 'nines' phrases, 'nine' doesn't refer to anything specific - it just means 'a lot'.