bil·low (bĭl′ō)
n.
1. A large wave or swell of water.
2. A great swell, surge, or undulating mass, as of smoke or sound.
v. bil·lowed, bil·low·ing, bil·lows
v.intr.
1. To surge or roll in billows.
2. To swell out or bulge: sheets billowing in the breeze.
v.tr.
To cause to billow: wind that billowed the sails.
[From Old Norse bylgja, a wave; see bhelgh- in Indo-European roots.]