Dressed to the Nines

One of English's oldest unsolved etymological puzzles. The earliest known appearance is in a 1719 poem by the Scottish writer William Hamilton, though not yet specifically about clothing. Guesses about where "nine" comes from range widely: some point to the number of yards of fine fabric needed for an elaborate outfit, others to the uniform of a specific 18th-century regiment, and one 19th-century lexicographer speculated it might be a corruption of "to thine eyne" (to your eyes). None of these has ever been backed up with real evidence. The most likely explanation etymologists offer is also the least satisfying: in several old "nines" phrases, the number nine doesn't refer to anything specific at all, it's simply an old, vague way of saying "a great deal," the way modern English uses "a hundred percent."