birthday suitbirthday suit
(plural birthday suits)
- (idiomatic) Nakedness; an exposure of a human's natural anatomy without clothing
- George embarrassed his aunt by answering the door in his birthday suit.
- Usually used in the phrase “in one's birthday suit", but sometimes seen alone; for example, as in “Guess what she was wearing? Her birthday suit!"
From the lack of clothes worn when one is born.