In a manner resembling brigands; in a lawless, plundering, or bandit-like way.
From brigandish (brigand + -ish) + -ly (adverb suffix). This creates the adverbial form of brigandish.
This rare adverb appears occasionally in historical novels to describe how colonial powers 'brigandishly seized' resources—it's a literary way of saying 'they robbed it like bandits.'
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