billow (n.) Look up billow at Dictionary.com
1550s, perhaps older in dialectal use, from Old Norse bylgja "a wave, a billow," from Proto-Germanic *bulgjan (cognates: Middle High German bulge "billow, bag"), from PIE *bhelgh- "to swell" (see belly (n.)).
billow (v.) Look up billow at Dictionary.com
1590s, from billow (n.). Related: Billowed; billowing.