The idiom: Chip on your shoulder means being angry about something that happened in the past.
The phrase appears to have originated with a 19th U.S. practice of spoiling for a fight by carrying a chip of wood on one’s shoulder, daring others to knock it off. A ‘chip’ can be defined as a piece of timber, or wood. Depending on the amount and size, timber can be quite heavy, and oftentimes people carry heavy things on their shoulders
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