It's a figure of speech.
Dressed to the Nines Meaning (according to The Phrase Finder)
"Dressed flamboyantly or smartly."
Personally, it feels good to "dress for dinner." It doesn't mean a full formal, unless that is what you want. I am a jeans girls myself and my husband is most comfortable in hunting apparel but it's nice & it's special to dress up a bit for dinner. Everyone feels good when they look good.
Carnival Cruise Line 1984
Diamond Princess-Baja 2007
Carnival Elation-Baja 2008
Star Princess-Alaska 2008
Star Princess-Alaska (again) 2009
HAL Oosterdam - New Year's 2009 into 2010
Carnival Spirit-Baja Birthday Cruise 2010
NCL Spirit - W Caribbean Bday Cruise Oct 2011
Island Princess -Alaska to Denali 2012
Coral Princess - Panama Canal 2013
2014 Booked - Azamara Barcelona to Monte
2015 Booked - Carnival Dream to Caribbean
"One prevailing theory concerning the origin of "dressed to the nines" surrounds the significance of the number nine. Some sources believe that the number 9 possessed a spiritual or cultural strength, much like the numbers 3,7 or 13 do today. For example, there were nine Muses said to inspire mankind's pursuit of the arts. If someone were described as "dressed to the nines", it could have been an outfit capable of impressing the nine Muses. The expression "to the nines" was actually in popular use before "dressed to the nines" came into vogue. It's entirely possible that the reference was meant as a tribute to the Muses or other significant figures.
Another theory states that women of the Middle Ages would often wear fashionable gloves as part of their formal wear. These gloves were said to contain nine buttons from wrist to elbow, so if a woman was said to be "dressed to the nines", she would be wearing her most formal evening wear. In a similar vein, the price for admission for theatrical performances often ranged from one shilling for a front row seat to nine shillings for the preferred balcony or box seating. Anyone who could afford the highest ticket prices might be considered "dressed to the nines" as opposed to the unwashed groundlings seated on the grass in front of the stage."
I got curious enough to do some research. Found the website where Parrotmom got her info. But, also found out that "to the nines" is such very old slang that it refers to anything, not just dress. It means "to the ultimate". It is so old a saying that it predates long gloves and a bunch of other "guesses". And, it originated as Old English. Which makes it impossible to know to origin. I was amazed that a bit of slang could survive that long!
I try to stay away from google because I can get lost in all the interesting but mostly useless info. And, so much of it does not come from credible sources. So, if I cared enough, I would then have to research sources, and on, and on. Rather chat about cruises!