gudgeon (n.1)
small freshwater fish, early 15c., from Middle French goujon, from Old French gojon (14c.), from Latin gobionem (nominative gobio), alteration of gobius, from Greek kobios, a kind of fish, of unknown origin. The figurative sense of "a credulous person" (one who will "bite" at "bait") is 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).