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.
2. What may be got without skill or merit.
Fish not, with this melancholy bait,
For this fool gudgeon, this opinion. --Shak.
3. A person easily duped or cheated. --Swift.
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.
6. (Naut.) A metal eye or socket attached to the sternpost to
receive the pintle of the rudder.
Ball gudgeon. See under Ball.