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.
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.