Archaic or dialectal; poisonous, venomous, or contaminated with 'atter' (poison or venom).
From Old English 'atter' (poison/venom) with the suffix '-y' or '-ery' indicating possession of a quality. Used in medieval texts to describe toxic or dangerous substances.
Medieval people used words like 'attery' to describe what we'd now call toxic or venomous—it's a reminder that English speakers have always needed ways to describe dangerous poisonous things.
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