Picture the open sea lifting into one huge slow wave: the water swells up into a round back, rolls forward, and another one comes along behind it. Billow keeps that shape wherever the word travels. Smoke pushes out of a window in fat grey rolls. A curtain fills with air and swells into the room. A long skirt puffs up when the wind gets under it. Three things stay the same each time: there is a lot of it, the shape is round rather than flat, and it is swelling or moving rather than sitting still. A thin line of smoke or a flat, still sheet is not a billow.
From the word’s history
English took billow from Old Norse bylgja, which simply meant a wave, and the Scandinavian languages still use the same word for one today. So the sea-wave is not one meaning among several; it is where the word starts, and the smoke, cloud and cloth uses carry that wave shape over to other things.
billow 來自古諾斯語 bylgja，意思就是「浪」；丹麥語 bølge、瑞典語 bölja 到今天還是這個意思（本包提供的 Wiktionary 詞源）。所以「大浪」不是它眾多意思裡的一個，而是它的起點。煙、雲、布的用法，都是把浪的形狀搬到別的東西身上。
Wiktionary entry for billow, etymology section supplied in this packet: from Middle English *bilwe, borrowed from Old Norse bylgja, from Proto-Germanic *bulgijō; cognates Danish bølge, Swedish bölja (wave).
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