A variant spelling of 'boulder'; a large rounded rock or stone, typically found on the ground surface.
From Middle English and Scandinavian sources (Swedish 'bullersten' and similar terms). The spelling 'bowlder' was common in 19th-century American English before 'boulder' became standard.
Bowlder' and 'boulder' compete in old American texts—the 'w' spelling actually made more etymological sense, but 'boulder' won out, showing how arbitrary spelling standardization can be!
Complete word intelligence in one call. Free tier — 50 lookups/day.