1610s, "water-worn rounded stone of medium or large size," variant of Middle English bulder ston "stone worn round, cobblestone" (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." Specific geological sense "large weather-worn block of stone standing by itself" is from 1813.
