Webster's English Dictionary

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billow

1. bil.low \'bil-(.)o-, -*(-w)\ n [prob. fr. ON bylgja; akin to OHG balg 
   bag - more at BELLY] 1: WAVE; esp : a great wave or surge of water  2: a 
   rolling mass (as of flame or smoke) like a high wave 
2. billow vi 1: to rise or roll in waves or surges  2: to bulge or swell 
   out (as through action of the wind)  : to cause to billow