To thunder loudly or intensely; to make sounds like thunder; to speak with a thunderous voice.
From 'be-' combined with 'thunder.' This intensifying prefix suggests thunder itself made more emphatic or comprehensive, following the Middle English verb-creation pattern.
When Shakespeare wanted to describe someone shouting, he could write 'he bethunders' rather than 'he shouts like thunder'—it's poetic compression at work. These 'be-' verbs let writers load actions with extra power and presence in a single word.
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