Green’s Dictionary of Slang

an enthusiast, a (knowledgeable) fan.

1915		O.O. McIntyre Day By Day in New York 26 Feb. [synd. col.] Simon Brentano, the most famous ‘buff’ in the world has passed on. He was head of a big Fifth Avenue publishing house, but his hobby was fires and their prevention [...] He was what New York’s foremen call a ‘buff’ — a ’bug on fires’.
1930		E.H. Lavine Third Degree (1931) 67: There are several varieties of police buffs. Some of them are merely amiable eccentrics.
1931		C.W. Willemse Behind The Green Lights 257: I knew him to be a ‘police buff,’ a ‘buff’ being a civilian who is enthusiastic about police work without being an actual criminologist. A fire ‘buff’ is just as big a ‘bug’.
1959		S. Bellow Henderson The Rain King 44: I’ve always been a sort of Africa buff.
1969		‘Iceberg Slim’ Pimp 67: She was a space buff all right.
1977		T. Berger Who is Teddy Villanova? 29: A real nut on the subject. A buff.
1988		T. Harris Silence of the Lambs (1991) 19: Ham radio enthusiasts and other personality-deficient buffs.
1996		L. Pettiway Honey, Honey, Miss Thang 60: I’m the television buff.
2000		Guardian G2 6 Jan. 19: Here’s a little puzzle for the cricket buffs.
2006		C.W. Ford Deuce’s Wild 187: I heard the sheik’s a jazz buff.
2010		A. Steinberg Running the Books 202: Deputy Mullin had told Gallo/Gallows that I was a ‘history buff’.