in birthday suit

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.
See also: birthday, suit
Farlex Dictionary of Idioms.

*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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • birthday
  • birthday suit
  • tell (one) what (one) can do with (something)
  • tell someone what to do with something
  • tell what to do with
  • scare the crap out of (one)
  • scare (one) silly
  • frighten/scare somebody out of their wits
  • frighten one out of wits