birthday suit, the suit in which Adam and Eve first saw each other, and were not ashamed.
He really looked as if Apollo had sent him a birthday suit, or a wedding-garment, and was witty and lively.
I had a fellow feeling for that Mazeppa party who was tied in his birthday suit to the back of a wild horse.
first attested 1730s, but probably much older. The notion is the suit of clothes one was born in, i.e., no clothes at all. Cf. Middle English mother naked "naked as the day one was born;" Middle Dutch moeder naect, German mutternackt.
To be “in one's birthday suit” is to be completely naked (as people are at birth).
What one is wearing when completely nude; the state of nakedness: Naked as a jaybird, with nothing on over his birthday suit (1730s+)