Because we were born naked, and that that was our ‘birth day,’ the phrase makes sense! But, a ‘Birthday Suit’ used to mean something much more literal!
As the story goes, some pre-1700 European kings would get all dressed up in new cloths on their birthdays and show off their outfits. (This still probably happens, but that’s beside the point!) This was a glimpse for most onlookers at the height of fashion for those times, and the new attire each year was commonly called a ‘Birthday Suit!’
Now, someone got it into their head to take the name “Birthday Suit’ and have it mean wearing nothing, just the opposite of its original use! This may have been a way to poke fun at the king, or to say up-yours to royalty altogether. Either way, the new meaning stuck!
Though there’s no way to prove it, some think Hans Christian Anderson’s tale, ‘The Emperor’s New Clothes,’ may have been a dig at royalty as well, using a double-play on the ‘Birthday Suit’ to illustrate a ruler who was too proud and townspeople who were too afraid to speak up!
Anyway, try to be careful in your birthday suit. Don’t ever steam clean it, and remember, if you’re over the age of five and you wear it in public, you’ll be arrested!