What is the origin of the term 'dressed to the nines'?
One theory is that it comes from the name of the 99th Wiltshire Regiment, known as the Nines, which was renowned for its smart appearance. There are a couple of problems with this suggestion, though. To begin with, the regiment's sartorial reputation seems to have dated from the 1850s, while the first recorded use of the phrase is from 1837. Secondly, dressed to the nines developed as an extension of the much earlier phrase to the nines, meaning 'to perfection, to the greatest degree': the 20-volume Oxford English Dictionary's first example of this earlier form dates back to 1719.
Why it should have been to the nines rather than to the eights, to the sevens, etc. remains unclear.