1. An engraved stamp which is pressed onto a piece of paper or other material to leave an impression on it. Doing this is called embossing.
  2. A raised nail or stud on a shield or armor. A shield featuring bosses is, straightforwardly enough, a bossed shield.
  3. A chieftain, foreman, manager, or other leader of a group; a honcho. As a verb, to give orders; hence, to boss around.

The confluence of these divergent definitions gives rise to various wags pointing out that "a boss is an ornamental stud."