gudgeon (n.1)

European small freshwater fish, early 15c., gojoun, from Old French gojon (14c.), from Latin gobionem (nominative gobio), alteration of gobius, from Greek kobios, a kind of fish, a word of unknown origin. They are easily caught, hence the figurative sense of "a credulous person" (one who will "bite" at "bait"), from 1580s.

gudgeon (n.2)

"pivot on the end of a beam," c. 1400, from Old French gojon "pin, peg, spike" (13c.), perhaps somehow an altered sense of gudgeon (n.1).

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Definitions of gudgeon from WordNet

gudgeon (n.)
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;
Synonyms: goby
gudgeon (n.)
small slender European freshwater fish often used as bait by anglers;
Synonyms: Gobio gobio