The Collaborative International Dictionary of English v.0.48:

Gudgeon \Gud"geon\ (g[u^]j"[u^]n), n. [OE. gojon, F. goujon,
   from L. gobio, or gobius, Gr. kwbio`s Cf. 1st Goby. ]
   1. (Zool.) A small European freshwater fish (Gobio
      fluviatilis), allied to the carp. It is easily caught and
      often used for food and for bait. In America the
      killifishes or minnows are often called gudgeons.
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   2. What may be got without skill or merit.
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            Fish not, with this melancholy bait,
            For this fool gudgeon, this opinion.  --Shak.
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   3. A person easily duped or cheated. --Swift.
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   4. (Mach.) The pin of iron fastened in the end of a wooden
      shaft or axle, on which it turns; formerly, any journal,
      or pivot, or bearing, as the pintle and eye of a hinge,
      but esp. the end journal of a horizontal.
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   6. (Naut.) A metal eye or socket attached to the sternpost to
      receive the pintle of the rudder.
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   Ball gudgeon. See under Ball.
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The Collaborative International Dictionary of English v.0.48:

Gudgeon \Gud"geon\, v. t.
   To deprive fraudulently; to cheat; to dupe; to impose upon.
   [R.]
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         To be gudgeoned of the opportunities which had been
         given you.                               --Sir IV.
                                                  Scott.
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WordNet (r) 3.0 (2006):

gudgeon
    n 1: small spiny-finned fish of coastal or brackish waters
         having a large head and elongated tapering body having the
         ventral fins modified as a sucker [syn: goby, gudgeon]
    2: small slender European freshwater fish often used as bait by
       anglers [syn: gudgeon, Gobio gobio]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "gudgeon":
   arbor, axis, axle, axle bar, axle shaft, axle spindle, axle-tree,
   babe, boob, chump, cinch, credulous person, cull, distaff, dupe,
   easy mark, easy pickings, fall guy, fish, fool, fulcrum, gimbal,
   gobe-mouches, greener, greenhorn, greeny, gull, hinge, hingle, hub,
   innocent, leadpipe cinch, mandrel, monkey, nave, oarlock, patsy,
   pigeon, pin, pintle, pivot, plaything, pole, prize sap, pushover,
   radiant, rowlock, sap, saphead, schlemiel, sitting duck, spindle,
   stooge, sucker, swivel, toy, trunnion, trusting soul, victim