Fig. naked; nude. (In the "clothes" in which one was born. *Typically: be ~; get [into] ~.) I've heard that John sleeps in his birthday suit.We used to go down to the river and swim in our birthday suits.
Nakedness, as in The doorbell rang, and here I was in my birthday suit. In 18th-century Britain this term originally referred to the clothes one wore on the king's birthday. Later it was jocularly transferred to bare skin, alluding to the condition of a newborn baby.