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.
<a href="https://www.etymonline.com/word/swelling">Etymology of swelling by etymonline</a>
Harper, D. (n.d.). Etymology of swelling. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/swelling