boulder: [13] Boulder is an abbreviated form of the original compound noun boulder-stone, which was a partial translation of a Scandinavian word which survives in Swedish dialect bullersten ‘large stone in a stream’. Sten is ‘stone’, of course, and buller is usually identified with Swedish buller ‘rumbling noise’, on the basis presumably of the sound of a stream gurgling over rocks. Boulder first appears on its own, outside the compound boulder-stone, in the 17th century.
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 *bhel- (2) "to inflate, swell" (see bole).
