From an Australian racehorse named Drongo, apparently after the bird (specifically, after the Spangled drongo, Dicrurus bracteatus (species)). The horse (foaled 1920, retired 1925) never won a race, and by transference anyone slow-witted or clumsy became a drongo.
Alternatively, from putative RAAF slang drongo (a recruit), similarly after the bird.
Another suggested derivation is the Scottish Gaelic drongair ("drunkard").
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