A person who has "a chip on his shoulder" is angry because of some thing that happened in the past. Example: "He lost his game this morning, and now he has a chip on his shoulder." It is easy for a person to get in a fight when he has a chip on his shoulder, because he is already angry about something else. Example: "Watch out for that guy, he's got a chip on his shoulder." To start a fight, men used to put chips of wood on their shoulder and challenge others to "try to knock it off". Example: "What's bothering that guy?" Reply: "Nothing. He's just got a chip on the shoulder." Example: "Tom had a tough time growing up, so he's got a bit of a chip on his shoulder." You can use the definite article ("the") which sounds more general ("a chip on the shoulder"), but more often people use the personal pronoun ("his", "her", "their") to say that that specific person has "a chip on his (her, their) shoulder."