He walked out of the bathroom in his birthday suit - obviously not expecting to find anyone in the flat.
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.