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.

"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).