"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."
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