A large South American boa or constrictor snake, possibly a regional name for anaconda or similar species.
From indigenous South American languages, likely Tupi or related Amazonian language family, with 'boigu' or similar meaning snake and 'acu' meaning large.
Boiguacu shows how Spanish and Portuguese colonizers in South America adopted indigenous words for dangerous animals they'd never seen before—rather than translating them, they just incorporated the native names into their own languages.
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