Old English swellan "grow or make bigger" (past tense sweall, past participle swollen), from Proto-Germanic *swelnan (cognates: Old Saxon swellan, Old Norse svella, Old Frisian swella, Middle Dutch swellen, Dutch zwellen, Old High German swellan, German schwellen), of unknown origin. Of emotions from late 14c., of music from 1749. Related: swelled; swollen; swelling.

c.1200, "a morbid swelling," from swell, v.. In reference to a rise of the sea, it is attested from c.1600; of music, by 1803. The meaning "wealthy, elegant person" is first recorded 1786, connected to the now-obsolete sense "pompousness, arrogance" (1724), both from the notion of "puffed-up" demeanor or behavior.

"fashionably dressed or equipped," 1810, from swell, n. in the "stylish person" sense. As "good, excellent," by 1897; as a stand-alone expression of satisfaction it is recorded from 1930 in American English.