Green’s Dictionary of Slang

(US) a damned fool.

1910		O. Johnson Varmint 103: ‘Nothing from the D F’s! Nothing at all?’ Dink burst into laughter, and laughed alone. The Roman stopped.
1915		P.W. Long ‘Semi-Secret Abbreviations’ in DN IV:iii 246: d.f. Damn fool.
1925		R. Lardner ‘Zone of Quiet’ in Coll. Short Stories (1941) 66: Maybe I’m a D.F. not to know, but would you tell me what a B.F. and a G.F. are?
1935		H.S. Truman letter 3 July in Ferrell Dear Bess (1983) 367: I’m just a d.f. I guess.