from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A form of challenge, in the same spirit as a medieval knight throwing down his gauntlet.
- n. A habitually combative attitude, usually because of a harboured grievance, sense of inferiority, or having something to prove.
- n. A tendency to take offence quickly.
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. Printed citations of this include the Long Island Telegraph, which on May 20th, 1830, printed: "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." (Wiktionary)