rocker n.

[to sway, move backward and forward] Middle English rokken "rock (a cradle), cause to sway back and forth; rock (someone) in a cradle," from late Old English roccian "move a child gently to and fro" in a cradle, which is related to Old Norse rykkja "to pull, tear, move," Swedish rycka "to pull, pluck," Middle Dutch rucken, Old High German rucchan, German rücken "to move jerkily."

The intransitive sense of "move or sway back and forth unstably" is from late 14c. For the popular music senses, see rock (v.2). Related: Rocked; rocking.

The earliest associations of the word were with slumber, rest, security. The sense of of "sway to and fro under some impact or stress" is from late 14c., especially of vessels in the waves (1510s); hence rock the boat in the figurative sense "stir up trouble" (1914). The sense of "swing to and fro in or as in a rocking chair" is by 1795.

"tidy, sophisticated teen" (usually contrasted with rocker), 1960, slang shortening of modern.

"to dance to popular music with a strong beat," 1948 (in song title "We're gonna rock"), from rock (v.1) in an earlier blues slang sense of "cause to move with musical rhythm" (1922); often used at first with sexual overtones, as in the 1922 song title "My Man Rocks Me (with One Steady Roll)". The sense developed in early 1950s to "play or dance to rock and roll music." Also see rock (n.2). Related: Rocked; rocking.

In reference to music, by 1938 as "to have a rocking rhythm;" by 1977 as "exhibit the characteristics of rock music." To rock out "enjoy oneself to rock music" is by 1968. Rocksteady, Jamaican pop music style (precursor of reggae), is attested from 1969.