Thugs or henchmen; hired muscle to intimidate or harm people; stupid or foolish people.
Possibly from 'goon' as a dialectal or slang term, though origin is unclear. May relate to 'gooney bird' or be influenced by 'goon' meaning a stupid person, emerging in American English by early 1900s.
The word 'goon' became famous through Max Fleischer cartoons where the Goon character was stupid and brutal—but long before cartoons, sailors used 'gooney' for albatrosses they thought were foolish birds!
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