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What is the origin of billow?

The noun billow, “a great wave or surge of the sea,” appears nearly 45 years before its derivative verb billow “to swell out, puff up, as by wind.” Billow appears in print pretty late in English, just after the middle of the 16th century, but it most likely comes from Old Norse bylgja “a billow,” from the Proto-Germanic root balg-, bulg– “to swell.” The root variant bulg– is the source of the Proto-Germanic noun bulgjan, the source of Old Norse bylgja. The root variant balg– forms the Proto-Germanic noun balgiz, source of Old English belg “bag.” Belg becomes beli in Middle English, and belly in modern English. Belgas, the plural of Old English belg, becomes belowes in Middle English, and bellows in modern English.