chip on one's shoulder

1. A belligerent attitude or grievance: “Joe really has a chip on his shoulder; every time I say something to him, he takes it the wrong way.” In the past, a young boy would place a wood chip on his shoulder and dare anyone to knock it off as a way of showing how tough he was.

Idioms and Phrases

A belligerent attitude or grievance, as in Mary is easily offended; she always has a chip on her shoulder. This term actually was defined in a newspaper article (Long Island Telegraph, May 20, 1830): “When two churlish boys were determined to fight, a chip would be placed on the shoulder of one and the other demanded to knock it off at his peril.” [Early 1800s]