People who play or listen to rock music, or curved supports on which something rocks back and forth.
From Middle English rokken meaning 'to move back and forth'. The musical sense developed in the 1950s from 'rock and roll', while the furniture sense comes from the rocking motion of chairs with curved bottom supports.
The word 'rockers' beautifully captures how the same motion that soothes babies in rocking chairs became the foundation of rebellious music that shook the world. It's fascinating that something so gentle and something so revolutionary share the same root concept of rhythmic movement.
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