1. An enthusiast, expert or aficionado. An American term which, in forms such as film-buff, opera-buff, etc., has become established in other English-speaking countries. The word is said (by American lexicographer Robert L. Chapman among others) to be inspired by the buff-colored raincoats worn by 19th-century New York firemen, later applied to watchers of fires, hence devotees of any activity.

'Having your life dragged through the popular press for scrutiny by a nation of voyeurs and trivia-buffs…'

(London Australasian Weekly, 4 September 1989)

2. The buff the nude. From the colour of (white) skin.