If you haven't read all of the other birthday-suit definitions, and this is the first definition you see, a birthday suit is when you don't have a article of clothing on you, you came out of your mom naked, so, I guess it's 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 wouldn't do that...)
1: "So, It's my birthday today."

2: "So, what are you gonna do to celebrate it?"

1: "Go see some people, do things I've wanted to do, and at the end of the day, just fall asleep with my birthday-suit..."

2: Whoa, you have a birthday-suit?

*1 looks at two with a serious look on his face,*

1: I do believe you have a birthday-suit of your own.
by *Insert_pseudonym* July 28, 2015
some trashy outfit you wear on your birthday or any other day of the year usually consists of a pair of panties and nothing else
Guy1: Hey man I saw Lisa in her birthdaysuit yesterday

Guy2: Really? What'd she look like?
by childplz January 07, 2011

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