goon

Shortened from gooney, from obsolete gony (“simpleton”), used circa 1580, of unknown origin. Perhaps a familiar term derived from Middle English gone, a variant of gome (“man, person”). Gony was applied by sailors to the albatross and similar big, clumsy birds (circa 1839). The term goon first carried the meaning "stupid person" (circa 1921). Compare Scots goni, guni (“a bogey, bugbear, hobgoblin”), dialectal Swedish gonnar (“elves, goblins”, plural).

Etymology 1, noun sense 1 ("hired thug"; circa 1938) is largely influenced by the comic strip character Alice the Goon from the Popeye series.

Etymology 1, noun sense 3 ("fool") was reinforced by the popular radio program, The Goon Show, starring Spike Milligan and Peter Sellers.

Etymology 1, noun sense 5 ("guard") was influenced by both etymology 1, noun sense 1 and etymology 1, noun sense 3, though not by The Goon Show reference, which arose about 10 years after WWII.

Perhaps diminutive slang for flagon or from Aboriginal English goom.