In the buff
In the buff means in the nude and dates from the 1600s. Buff here is the colour buff, a light brown, and derives from what was known as a buff-coat which was a light brown or buff leather jerkin commonly worn by soldiers. At first, to be in buff simply meant wearing one of these leather jerkins. By the early 1600s, it came to mean naked purely because of the light brown colour, which resembled the colour of naked skin.