rockabilly n.

1823, "action of rocking; a movement to and fro," from rock (v.1). As short for rock and roll, by 1957; but the sense of "musical rhythm characterized by a strong beat" is from 1946, in blues slang (Mezz Mezzrow, "Really the Blues"). Rock star is attested by 1966.

"southern Appalachian person," by 1892, from hill (n.) + Billy/Billie, popular or pet form of William. In reference to a type of U.S. folk music, first attested 1924.

I would hate to see some old railroad man come here and take my job, and then, I don't think it is right to hire some Hill Billy and give him the same right as I just because he was hired the same time I was. [The Railroad Trainmen's Journal, vol. ix, July 1892]

In short, a Hill-Billie is a free and untrammelled white citizen of Alabama, who lives in the hills, has no means to speak of, dresses as he can, talks as he pleases, drinks whiskey when he gets it, and fires of his revolver as the fancy takes him. [New York Journal, April 23, 1900]

In Scott's collection of Border ballads, billie is a frequent term of address or intimacy, "comrade, companion, a brother in arms," "a term expressive of affection and familiarity" also "a brother; a wooer of a woman," and generally "a young man" [Jamieson, 2nd edition]. It is said to be a variant of bully (n.) in its old sense of  "sweetheart," also "fine fellow."