A person who plays or enjoys rock music, or a curved piece of wood or metal that allows something to sway back and forth.
From 'rock' (the music genre, originating in 1950s America) plus the agent suffix '-er' meaning 'one who.' The original rocking chair meaning comes from the curved runners that 'rock' or sway.
The word 'rocker' captures two totally different things—a wild musician and a chair that moves—because both involve movement and rhythm. Rock music itself was called that because it had a steady beat you could literally rock back and forth to!
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