USAAF influence WWII? "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.
AMERICANISMS; The English of the New World", M. Schele De Vere, LLD
Of all Dutch words familiar to our ear, none has acquired a wider circulation and a stronger hold on our social system than the term boss, derived from the Dutch baas. It had, originally, with us as in its native land, the primitive meaning of “ master,” overseer, or superior of any kind, and retains it to this day in a large measure. Even now a boss shoemaker, or a boss bricklayer means the head of a gang of workmen, who deals their work out to them, and pays their wages, as an English master does to his workmen and apprentices. In this sense it is, even in England, now the cant term, if nothing more, with all mechanics, and can boast high antiquity for such a meaning, since as early as 1679, M. Philipse wrote: “Here they had their first interview with the female boss or supercargo of the vessel,” (Early Voyage to New Netherlands). strangely foreshadowing the “Advanced Female" of the New World. For the proud Yankee, from the beginning. disliked calling any man his master, a word which, as long as slavery existed, he thought none but a slave should employ; and as the relation between employer and employed required a. word, the use of boss instead of master, was either coined or discovered. Thus the word became early a part of the language in Northern and Western States, and Lord Carlisle could enjoy the naive question propounded to him by his stage-driver: “I suppose the Queen is your boss, now ?” In the same sense the slang loving New York Herald said, in speaking of the Pope: “Rothschild. refused to let him have any (money). The fact is, Rothschild is the pope and boss of all Europe.” It is curious that the word has actually found its way into French also, although only as a cant term; for M. Francisque Michel, in his Dictionnaire d' Argot, has : Beausse, un riche bourgeois, terme des voleurs Flamands. It made its way Southward, in America, but very slowly, and reached Pennsylvania only about 1852, with the construction of railways and canals. Since the emancipation of slaves in the South, the negroes also have become too proud to continue their old mode of address, and substitute for it the Northern boss, so that the word may fairly be said to be in universal use all over the Union. It has even been turned into a verb, and to boss is quite a common expression, meaning to direct anything, from bossing a job, that is, to contract and superintend it, to bossing the house, which means in the case of the husband or the wife, as Providence may direct, to rule and manage it. So familiar has the word become, that we are told of a child not five years old put into a corner for quarrelling, who wished to charge his sister with being the aggressor, and said: “ I did not boss the job, it was sister.” ( S. S. Haldeman.) Thus the Dutchman is master in the land after all.