Birthday-suit

[Rihanna]

If you havent read all of the other birthday-suit definitions, and this is the first definition you see, a birthday suit is when you dont have a article of clothing on you, you came out of your mom naked, so, I guess its only reasonable to walk around naked on your birthday as well, (Around private places, no one wants to see your private-parts out in public, hell, I wouldnt do that...)