Old English swellan "grow or make bigger" (past tense sweall, past participle swollen), from Proto-Germanic *swellanan (source also of Old Saxon swellan, Old Norse svella, Old Frisian swella, Middle Dutch swellen, Dutch zwellen, Old High German swellan, German schwellen), which is of unknown origin, perhaps a substratum word. Of emotions from late 14c., of music from 1749. Related: swelled; swollen; swelling.
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Definitions of swelling from WordNet
an abnormal protuberance or localized enlargement;