Present participle of billow; swelling and surging like waves; as an adjective, currently rising and falling in waves.
From 'billow' plus the present participle suffix '-ing,' creating a continuous form from the Old Norse-derived verb.
Billowing appears constantly in historical fiction and poetry because the '-ing' form creates a sense of ongoing, dramatic motion—'billowing smoke' sounds so much more cinematic than 'wavy smoke.'
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