Green’s Dictionary of Slang

1. (US gay) a synon. for motherfucker n.

1963

J. Rechy City of Night 335: Father-fuckers! I’ll take you on together or alone! Prove to Me what big men you are!

2. (US black) a positive epithet.

1982		G. Tate ‘Beyond the Zone of the Zero Funkativity’ in Flyboy in the Buttermilk (1992) 41: Like I said upfront, however, Computer Games is one signifying furthermucker 20 times over.
1983		G. Tate ‘Electric Miles’ in Flyboy in the Buttermilk (1992) 82: These furthermuckers were making it up night after night on the road, making new music every time.