American colonists borrowed a Dutch word for uncle to avoid saying master.
The word boss entered American English in the 1640s, borrowed from the Dutch baas, meaning master.1 Dutch settlers in New Amsterdam, the colony that would become New York, used baas as a term of authority. The Middle Dutch baes originally meant "master of a household" or "friend," and may trace further back to Old Dutch baso, meaning uncle or kinsman.2
The word's adoption in American English was not accidental. In the colonies, master was the standard English term for a person in authority. As the institution of slavery made that word increasingly uncomfortable, settlers reached for an alternative.
The Dutch form baas appears in English records as early as 1625, first as the standard title for a Dutch ship's captain.3 By the early nineteenth century, boss had spread well beyond the former Dutch colonies. Washington Irving used the spelling "boss" in 1806.4 James Fenimore Cooper condemned it in the 1830s as a barbaric vulgarity, which had no effect on its adoption.
The word carried a democratic charge. To call someone your boss instead of your master was to acknowledge their authority while preserving a fiction of equality. The relationship was economic, not feudal. You could, in theory, leave.
In South African English, the same Dutch root produced baas with a very different connotation, used by non-white workers to address European overseers during apartheid.5 The same word that softened hierarchy in one country reinforced it in another.
By the late nineteenth century, boss had acquired a political meaning in the United States. The Tammany Hall "bosses" of New York City used the word to describe leaders who controlled party machinery through patronage and favor. The word that began as a polite substitute for master had become a synonym for unchecked power exercised without formal title.