in birthday suit
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in (one's) birthday suit
Completely naked. Alludes to the fact that one is born without clothing. I have a recurring nightmare where I'm walking through school in my birthday suit. As kids, we had no problem going swimming together in our birthday suits.
*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.