Boss has two distinct meanings, and though one might seem to be able to draw a connection between them—they both refer to something that stands out among others—they have different etymological origins.
The earlier, and less prevalent, meaning is “protuberance,” from a Latin word of uncertain origin by way of the Old French term boce, meaning “swelling.” The word usually pertains to the protruding central component of a shield, which helped the bearer ward off blows from an opponent. The derived verb emboss and adjective embossed, formed by the attachment of a prefix meaning “in” or “into,” refers to raised ornamentation or inscription in general.
Boss also refers to the hub of a propeller, a projecting stone block in a wall or other architectural element, a similar naturally occurring feature in geology, or a pad used in some crafty endeavors such as ceramics and glassmaking.
Interestingly, bocle, a variation of the Old French precursor, is also the source of buckle, both a word referring to a ring of metal (or other material) used for fastening or the act of fastening something to it and a word meaning “bend out of shape,” and buckler, the name of a small, round shield (and the resulting term swashbuckler, referring to a literary genre involving feats of derring-do; the term is perhaps inspired by the image of a buckler-wielding hero).
The dominant meaning of boss, that of “supervisor,” is from the Dutch word baas, meaning “master” (and possibly originally meaning “uncle”). The term was used to refer to the person in charge of a Dutch ship. (In English, the technically correct word is master, though it has been largely supplanted by captain.) The adoption of the Dutch term into English may be influenced by the assimilation of Dutch colonies in North America into the British colonies in the 1600s, with the attendant infiltration of Dutch vocabulary into American English, but it also may reflect an effort among Americans after the Revolutionary War to distance themselves from a term used by the English, and later from one that slave owners expected their slaves to use (though some slaves addressed their owners as “boss” as well).
Boss also entered the language as a slang adjective meaning “excellent.” This first occurred in the late 1800s, but it returned to use in the 1950s and again in the 1970s, in similar usage as a synonym for cool.
Another adjectival form is bossy, originally from the sense of “swelling” and meaning “projecting” or “decorated with bosses” but subsequently assigned to the later meaning, referring to someone who is domineering. And although the plodding, stubborn, mooing behavior of cattle may be interpreted as being bossy, that word as an endearing proper name for a cow is unrelated; that term stems from the Latin word bos, meaning “cow,” from which beef and bovine are derived.