|informal, jocular a state of total nakedness, as at birth|
To be “in one's birthday suit” is to be completely naked (as people are at birth).
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.