Birthday: Suit

The phrase is a popular English idiom used as a humorous euphemism for nakedness . It literally refers to the "clothing" a person wears on the day they are born—which is nothing at all. The Origin and Meaning

The term stems from the simple fact that infants are born without clothes. By calling nudity a "suit," speakers use irony to describe a state of undress as if it were a formal garment. Historically, the word "birthday" in this phrase referred specifically to the actual day of one's birth, a sense of the word that is now rare outside of this specific idiom. Common Usage and Context birthday suit