Origin and history of swollen

Middle English swellen, from Old English swellan "grow in bulk, become bigger" (intransitive, past tense sweall, past participle swollen), from Proto-Germanic *swellanan, *swallejanan (source also of Old Saxon swellan, Old Norse svella, Old Frisian swella, Middle Dutch swellen, Dutch zwellen, Old High German swellan, German schwellen "make swell"), which is of unknown origin. "Most likely a substratum etymon" [Boutkan].

By c. 1200 as "move or spread upward" (of the sea, etc.), also "become markedly enlarged." In reference to shape, "protuberate, bulge out, belly," as a sail in the wind or the middle of a cask, from 1670s.

In reference to emotions, pride, etc., "rise and grow," from late 14c.; of music from 1749. Transitive sense, "make larger in bulk" (also size, amount, number of) is from c. 1400. Related: swelled; swollen; swelling.