Synonyms for In one's birthday suit:

adj.

all (adjective)
mother-naked, in your birthday suit, naked as the day you were born, naked as the day one was born.
bare (adjective)
unclothed.

n.

in one's birthday suit (noun)
mother-naked, unclothed, naked as the day one was born, naked as the day you were born, in your birthday suit.