billow (n.) Look up billow at Dictionary.com
"a great wave or surge of the sea," 1550s, perhaps older in dialectal use (but not recorded in Middle English), from Old Norse bylgja "a wave, a billow," from Proto-Germanic *bulgjan (source also of Swedish bölja, Danish bölge "a billow," Middle High German bulge "a billow; a bag"), from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell."
billow (v.) Look up billow at Dictionary.com
"to rise or roll in large waves," 1590s, from billow (n.). Related: Billowed; billowing.