in/wearing your 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

in your birthday suit

INFORMAL
If you are in your birthday suit, you are not wearing any clothes. I could go out there in my birthday suit and nobody would even notice. Note: This expression is used humorously.
See also: birthday, suit

in your birthday suit

naked. humorous
See also: birthday, suit

in/wearing your ˈbirthday suit

(informal, humorous) wearing no clothes; naked: The towel fell off, and there he was in his birthday suit!
See also: birthday, suit, wearing