in your birthday suit
not wearing any clothes – used humorously

[TahlilGaran] Dictionary of Contemporary English ▲

naked not wearing any clothes or not covered by clothes - used especially when this seems rather shocking:
a naked man
He got out of bed naked, and answered the telephone.
his naked chest
have nothing on/not have anything on to not be wearing any clothes. This phrase is very commonly used in everyday English instead of saying that someone is naked:
Can you wait a minute? I've got nothing on!
The little boy didn’t have anything on when he answered the door!
bare used about feet, legs, arms etc that are not covered by clothes:
The sand was too hot to walk on in bare feet.
nude naked – used especially when talking about naked people in paintings, films etc:
a nude portrait of his wife
undressed [not before noun] not wearing any clothes, especially because you have just taken them off in order to go to bed, have a bath etc:
Sara was undressed and ready for bed but Jenny was fully clothed.
Wearily she got undressed and stepped into the shower.
topless if a woman is topless, she is not wearing any clothes on the upper part of her body, so that her breasts are not covered:
a topless waitress
in the nude not wearing any clothes:
Men and women swam together in the nude and thought nothing of it.
in your birthday suit informal humorous not wearing any clothes:
He likes to sleep in his birthday suit. He says pyjamas are uncomfortable.
go commando informal humorous to not wear any underwear:
In summertime, he likes to go commando.
in the buff informal not wearing any clothes - a very informal use:
a beach where you can sunbathe in the buff

[TahlilGaran] English Thesaurus ▲

in your birthday suit
in (your) birthday suit
[humorous]
not wearing any clothes.
He walked out of the bathroom in his birthday suit - obviously not expecting to find anyone in the flat.

[TahlilGaran] English Idioms Dictionary ▲