Green’s Dictionary of Slang

(US, also billy-maria) a hillbilly; also as adj.

1908		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:iv 291: billy-maria, n. A populite, a country cracker.
1951		Green & Laurie Show Biz from Vaude to Video 547: The folk song vogue was such that anybody and everybody did ’billy tunes as well as the livelier tempos.
1989		H. Rawson Dict. of Invective (1991) 279: Other generics include: [...] Billy, as in hillbilly.
1991		D. Simon Homicide (1993) 420: The billies swarmed into the red brick [...] rowhouses.
1997		Simon & Burns Corner (1998) 317: Three black men and a white girl held hostage in the street by a red-faced ’billy.