To have a "chip on the shoulder" is an American idiom first recorded in the 19th century. Someone who has a chip on his or her shoulder is combative, prickly and ready to take offense. This phrase can describe either someone who is in a bad mood and looking for a fight or someone who is pugnacious by nature. It is often associated with resentment and a feeling of grievance or insecurity.
The origin of the phrase "chip on the shoulder" seems to lie in social customs from 19th-century America. During this period, placing a chip of wood or small stone on one's shoulder was a way of challenging someone else to fight. By knocking the chip off, an opponent accepted the challenge.
In the 1820s and 1830s, the expression appears in literal terms in American writing. The custom of placing a chip or stone on the shoulder as a challenge appears to be known from this period. In 1855, the "Weekly Oregonian" used the first recorded instance of the phrase in its figurative sense, to describe someone looking for a quarrel rather than an individual who literally had a chip of wood on his shoulder.
An alternative explanation for the phrase is that it dates to 18th-century England. According to this explanation, dockyard workers and employers quarreled over a traditional practice by which carpenters were allowed to take surplus pieces of wood, or "chips," home. An account of a 1756 dockyard strike includes a description of workers leaving the dockyard "with the chips on their shoulders." This phrase is very close to the expression "a chip on the shoulder," but there is no record of the phrase being used in a figurative sense until the 19th century; it does not appear again in Britain until the 1930s. The appearance of the expression in this report is thus likely to be a coincidence.
The idiom "chip on the shoulder" is an example of a phrase which has outlived the metaphor that originally motivated it. Speakers of American idiomatic English all know that a person with a chip on the shoulder is a person who is easily antagonized and takes any criticism or contradiction as a provocation. However, the practice of placing an actual chip of wood on the shoulder as a challenge to a fistfight has long since died out, making the phrase puzzling to anyone who does not know its history.