chip on one's shoulder

Meanings

1. A form of challenge, in the same spirit as a medieval knight throwing down his gauntlet.
2. A habitually combative attitude, usually because of a harbored grievance, a sense of inferiority, or a desire to prove something.
3. A tendency to take offence quickly.

Etymology

The saying originated during the 19th century in the United States, where people wanting a physical fight would carry a chip of wood on their shoulder, daring others to knock it off.