A small freshwater fish, or the socket or bearing on which something (like a door or boat rudder) rotates; also slang for a gullible person.
From Old French 'goujon' (fish), from Latin 'gobio,' likely related to 'gobius' (goby). The meaning expanded to the mechanical pivot because the fish's small rounded shape resembled these pin-like hardware pieces.
A gudgeon is one of those words that somehow ended up in completely different contexts—it's a real fish, a real piece of machinery, AND a word meaning a fool—because the fish's shape gave mechanics the perfect name for a tiny pivot pin.
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