Hey, that was more interesting than I expected. It looks like "boss" is in fact a loan from Dutch rather than a cognate:
> 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.
