Have a chip on your shoulder

To harbor resentment, feel defensive, or have a confrontational attitude, often due to feeling unfairly treated.

This phrase originated in 19th-century America, where placing a wood chip on one's shoulder and daring others to knock it off was a way to provoke a fight. The practice was documented as early as the 1830s and became a metaphor for someone walking around with a combative attitude, ready to take offense.