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.
in your birthday suit(humorous)
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.