Making a loud, deep, rolling sound like thunder, or impressively great and powerful in general.
From Old English 'thunor' (the thunder god) plus the present participle '-ing.' The word describes the sound itself and metaphorically describes anything powerful and loud.
Thunder scared ancient peoples so much they named their gods after it—Thunor (Norse), Thor (Scandinavian), Jupiter Tonans (Roman). When we say something is 'thundering,' we're using that ancient primal fear to express power!
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