What word or phrase does your mother always say?
1956, from noun sense of rock (n.2) in the music sense + second element abstracted from hillbilly music. One of the first uses is in a "Billboard" item about Johnny Burnette's "Lonesome Train."
: showing up at rockabilly dances and clubs in full '50s regalia
A blend of black rhythm and blues with white hillbilly music (1956+)