Birthday Suit Page

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

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 birthday suit