The Collaborative International Dictionary of English v.0.48:

Rocker \Rock"er\, n.
   1. One who rocks; specifically, one who rocks a cradle.
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            It was I, sir, said the rocker, who had the honor,
            some thirty years since, to attend on your highness
            in your infancy.                      --Fuller.
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   2. One of the curving pieces of wood or metal on which a
      cradle, chair, etc., rocks.
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   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.
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   5. A chair mounted on rockers; a rocking-chair.
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   6. A skate with a curved blade, somewhat resembling in shape
      the rocker of a cradle.
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   7. (Mach.) Same as Rock shaft.
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   Rocker arm (Mach.), an arm borne by a rock shaft.
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The Collaborative International Dictionary of English v.0.48:

Rock shaft \Rock" shaft`\ [Cf. Rock, v. i.] (Mach.)
   A shaft that oscillates on its journals, instead of
   revolving, -- usually carrying levers by means of which it
   receives and communicates reciprocating motion, as in the
   valve gear of some steam engines; -- called also rocker,
   rocking shaft, and way shaft.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cradle \Cra"dle\ (kr[=a]d'l), n. [AS. cradel, cradol, prob. from
   Celtic; cf. Gael. creathall, Ir. craidhal, W. cryd a shaking
   or rocking, a cradle; perh. akin to E. crate.]
   1. A bed or cot for a baby, oscillating on rockers or
      swinging on pivots; hence, the place of origin, or in
      which anything is nurtured or protected in the earlier
      period of existence; as, a cradle of crime; the cradle of
      liberty.
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            The cradle that received thee at thy birth.
                                                  --Cowper.
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            No sooner was I crept out of my cradle
            But I was made a king, at nine months old. --Shak.
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   2. Infancy, or very early life.
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            From their cradles bred together.     --Shak.
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            A form of worship in which they had been educated
            from their cradles. --Clarendon.
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   3. (Agric.) An implement consisting of a broad scythe for
      cutting grain, with a set of long fingers parallel to the
      scythe, designed to receive the grain, and to lay it
      evenly in a swath.
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   4. (Engraving) A tool used in mezzotint engraving, which, by
      a rocking motion, raises burrs on the surface of the
      plate, so preparing the ground.
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   5. A framework of timbers, or iron bars, moving upon ways or
      rollers, used to support, lift, or carry ships or other
      vessels, heavy guns, etc., as up an inclined plane, or
      across a strip of land, or in launching a ship.
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   6. (Med.)
      (a) A case for a broken or dislocated limb.
      (b) A frame to keep the bedclothes from contact with the
          person.
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   7. (Mining)
      (a) A machine on rockers, used in washing out auriferous
          earth; -- also called a rocker. [U.S.]
      (b) A suspended scaffold used in shafts.
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   8. (Carp.) The ribbing for vaulted ceilings and arches
      intended to be covered with plaster. --Knight.
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   9. (Naut.) The basket or apparatus in which, when a line has
      been made fast to a wrecked ship from the shore, the
      people are brought off from the wreck.
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   Cat's cradle. See under Cat.

   Cradle hole, a sunken place in a road, caused by thawing,
      or by travel over a soft spot.

   Cradle scythe, a broad scythe used in a cradle for cutting
      grain.
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WordNet (r) 3.0 (2006):

rocker
    n 1: an attendant who rocks a child in a cradle
    2: a performer or composer or fan of rock music [syn: rocker,
       rock 'n' roll musician]
    3: a teenager or young adult in the 1960s who wore leather
       jackets and rode motorcycles
    4: a chair mounted on rockers [syn: rocking chair, rocker]
    5: a trough that can be rocked back and forth; used by gold
       miners to shake auriferous earth in water in order to
       separate the gold [syn: rocker, cradle]
    6: an ice skate with a curved blade
    7: a curved support that permits the supported object to rock to
       and fro

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "rocker":
   burin, clarifier, colander, cradle, cribble, die, engraving tool,
   etching ball, etching ground, etching needle, etching point,
   filter, graver, intaglio, lixiviator, metronome, needle,
   oscillator, pendulum, percolator, point, punch, purifier, refiner,
   refinery, riddle, rocking chair, rocking stone, scorper, screen,
   seal, seesaw, shuttle, shuttlecock, sieve, sifter, stamp, strainer,
   style, swing, teeter, teeter-totter, teeterboard, teetery-bender,
   vibrator, winnow, winnowing basket, winnowing fan,
   winnowing machine