Green’s Dictionary of Slang

(US) a mediocrity, whether a person or thing.

1908		D.G. Phillips Susan Lenox I 250: His vanity had got savage wounds from the hoots and the ‘Oh, bite it off, hamfat,’ which had greeted his impressive lecture on the magic lantern pictures.
1914		T.A. Dorgan Indoor Sports 20 May [synd. cartoon] Listen Mr Hampfatt, you’ve been telling me of big time jobs you’ve been promised for the last three weeks.
1938		N.Y. Amsterdam News 12 Mar. 17: The Harlem Hamfats grind out the tune on myriad Harlem piccolos (i.e. juke boxes).
1946		Mezzrow & Wolfe Really the Blues 49: A lot of beat-up old hamfats who sang and played a commercial excuse for the real thing.
1958		Hughes & Bontemps Book of Negro Folklore 484: hamfat: A phoney, a worthless character, a poser. I hate hamfats myself.
1986		D. Barker Life in Jazz 45: There was on the scene a large group of musicians [who] never seemed to improve on their instruments [...] They played limited repertories: simple songs, nothing complicated or exciting. I have watched these groups of musicians (referred to as ‘ham fats’ because they were careless and indifferent).