chip on one's shoulder

noun3

1. •
   A habitually combative attitude, usually because of a harbored grievance, a sense of inferiority, or a desire to prove something.

   • The city of Herculaneum […] held its neighbors in hearty contempt, like the youth who has suddenly found his man's strength, and parades round with a chip on his shoulder. — 1906
2. •
   A tendency to take offence quickly.
3. •
   A form of challenge, in the same spirit as a medieval knight throwing down his gauntlet.

   • ‘Oh! if I only could get him to knock a chip off my shoulder, and so get round the law, I would give him one of the soundest thrashings he ever had.’ — 1830