You use the term ‘In The Buff’ to indicate that someone is naked.
Example of use: “I was horribly embarrassed when I accidentally walked in on Peter while he was in the buff.”
Interestingly,the origin of the term ‘in the buff’ refers to a buff-colored coat of lightweight leather worn by English soldiers until the 17th century. Originally, being “in the buff” was a reference to wearing this coat. The latter meaning is an allusion to the color of light-brown to yellowish human skin, and was first seen in the 1602 work by Thomas Dekker, Satiro-mastix or the Untrussing of the Humorous Poet: “No, come my little cub, doe not scone mee because I go in Stage, in Buffe, heer’s veluet too.” At the time, “in stag” also meant “naked.”