boulder (n.) Look up boulder at Dictionary.com
1670s, variant of Middle English bulder (c. 1300), from a Scandinavian source akin to Swedish dialectal bullersten "noisy stone" (large stone in a stream, causing water to roar around it), from bullra "to roar" + sten "stone." Or the first element might be from *buller- "round object," from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell."