Where does “swelli” come from?

swelli (Old High German) comes from Proto-Germanic sulī, from Proto-Indo-European sēl- — to calm, quiet, be favourable.

Ancestry of “swelli”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	sulī	balk, bar, threshold, sill
2	Proto-Indo-European	sēl-	to calm, quiet, be favourable

swelli — Etymology, Origin & Word History | Etymology Explorer