naked, in the buff*, in the raw*, in one's birthday suit*, in a state of undress.
See nude in Webster's New World Roget's A-Z Thesaurus II
adjective
Not wearing any clothes:
au naturel, bare, naked, unclad. (Chiefly British) starkers. Idioms: in one's birthday suit, in the altogether (or buff) (or raw), naked as a jaybird, stark naked, without a stitch. See put on, show