"overseer, one who employs or oversees workers," 1640s, American English, from Dutch baas "a master," Middle Dutch baes, of obscure origin. If original sense was "uncle," perhaps it is related to Old High German basa "aunt," but some sources discount this theory.
The Dutch form baas is attested in English from 1620s as the standard title of a Dutch ship's captain. The word's popularity in U.S. may reflect egalitarian avoidance of master (n.) as well as the need to distinguish slave from free labor. The slang adjective meaning "excellent" is recorded in 1880s, revived, apparently independently, in teen and jazz slang in 1950s.
"protuberance, button," c. 1300, from Old French boce "a hump, swelling, tumor" (12c., Modern French bosse), from either Frankish *botija or Vulgar Latin *bottia, both of uncertain origin.
"be master or manager of, to order and direct as a boss," 1856, from boss (n.1). Related: Bossed; bossing.
c. 1400, "to swell out; to beat or press into a raised ornament," from boss (n.2). From 1620s as "to furnish with bosses." Related: Bossed; bossing.