The Collaborative International Dictionary of English v.0.48:

Rocker \Rock"er\, n.
   1. One who rocks; specifically, one who rocks a cradle.
      [1913 Webster]

            It was I, sir, said the rocker, who had the honor,
            some thirty years since, to attend on your highness
            in your infancy.                      --Fuller.
      [1913 Webster]

   2. One of the curving pieces of wood or metal on which a
      cradle, chair, etc., rocks.
      [1913 Webster]

   3. Any implement or machine working with a rocking motion, as
      a trough mounted on rockers for separating gold dust from
      gravel, etc., by agitation in water.
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   4. A play horse on rockers; a rocking-horse.
      [1913 Webster]

   5. A chair mounted on rockers; a rocking-chair.
      [1913 Webster]

   6. A skate with a curved blade, somewhat resembling in shape
      the rocker of a cradle.
      [1913 Webster]

   7. (Mach.) Same as Rock shaft.
      [1913 Webster]

   Rocker arm (Mach.), an arm borne by a rock shaft.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rocker arm
    n 1: a lever pivoted at the center; used especially to push a
         valve down in an internal-combustion engine [syn: rocker
         arm, valve rocker]