Rockabilly is a kind of popular music from the 1950s in the American South. It takes the beat and electric guitar of early rock and roll and mixes them with country singing — twangy voices, slap bass, sometimes a little hiccup in the vocal. Elvis Presley’s very early records are the picture most people have. It is a music-fans’ word; you would not use it for any rock song, only this old mix. Though actually, the “billy” bit is not a man named Billy: it comes from hillbilly, an old and not very polite word for country people in the hills, and for their music.