mid-13c., "rich cloth" (often, but not necessarily, bright red), from a shortened form of Old French escarlate "scarlet (color), top-quality fabric" (12c., Modern French écarlate), which, with Medieval Latin scarlatum "scarlet, cloth of scarlet," Italian scarlatto, Spanish escarlate often is said to be from a Middle Eastern source, but perhaps is rather from a Germanic source akin to Old High German scarlachen, scharlachen (c. 1200), from scar "sheared" + lachen "cloth."
In English as the name of a color, attested from late 14c. As an adjective from c. 1300. Scarlet lady, etc. (Isaiah i.18, Revelation xvii.1-5) is from notion of "red with shame or indignation." Scarlet fever is from 1670s, so called for its characteristic rash. Scarlet oak, a New World tree, attested from 1590s. Scarlet letter traces to Hawthorne's story (1850). German Scharlach, Dutch scharlaken show influence of words cognate with English lake (n.2).