Also, dressed to the nines. Elaborately attired, as in For the opening of the restaurant she was dressed to kill, or At the opera everyone was dressed to the nines. The first of these hyperbolic expressions dates from the early 1800s and uses kill in the sense of “to a great or impressive degree.” The phrase to the nines in the sense of “superlative” dates from the late 1700s and its original meaning has been lost, but the most likely theory is that it alludes to the fact that nine, the highest single-digit numeral, stands for “best.” Also see gussied up.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.