Plural of billow (large waves or surges of any substance), or third-person singular present tense of the verb billow.
Either the plural noun form from Old Norse 'bylgja,' or the verb form showing the present tense with a third-person singular subject.
In Romantic poetry, billows of smoke, billows of fabric, and billows of emotion all used the same word—showing how 'billow' became the ultimate word for any kind of dramatic surging motion.
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