Billow \BIL-oh\ , verb;
1. To rise or roll in or like a great wave; surge
2. To swell out, puff up, etc. as by the action of wind
3. To make rise, surge, swell
1. A great wave or surge of the sea
2. Any surging mass
Billow dates to the 1550's, though it may be older in dialectal use. It comes from Old Norse bylgja ("a wave") from Proto-Germanic *bulgjan, which derives from Proto-Indo-European *bhelgh- ("to swell"). Does the PIE word look familiar? It should, it's also the forebear of belly.