re: Where did "chip on shoulder" come from?

None of the answers so far have been correct. The phrase actually arose in the middle ages. Folks walked around covered in fresh shite all the time. The more "well to do" folks, the upper crust, if you will, were able to procure older, hardened cow shite chips. They would wear them on their shoulders so everyone could see them. It also showed that they had so much shite that they could waste some by wearing it. The phrase originally referred to someone that was snooty. Those folks tended to have bad attitudes, so over time, the meaning evolved to what we know it as today.