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Unknown. Compare later haglet (“shearwater, petrel”). Attested (as hagden) from the 17th century.
Etymology from Wiktionary, CC BY-SA 4.0.
hagdon
Today it means: Any member of any of several species of sea birds of the genus Puffinus, especially, of species Ardenna gravis (syn. Puffinus gravis, Puffinus major; great shearwater) and of species Puffinus stricklandi (black hagdon or sooty shearwater). Full definition →