in birthday suit

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*in one's birthday suit

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.
See also: birthday, suit

in your birthday suit

not wearing any clothes
Usage notes: Babies are naked at the time of their birth.
He walked out of the bathroom in his birthday suit - obviously not expecting to find anyone in the flat.
See also: birthday, suit