the.com/chip on shoulder

means A lingering resentment or defensiveness that makes someone quick to take offense or pick a fight, as if provoked by an old, unhealed slight.

from The phrase comes from an old American and British boyhood custom: a boy spoiling for a fight would balance a wood chip on his shoulder and dare anyone to knock it off — accepting the dare meant accepting the fight. The practice is recorded in print by the early-to-mid 1800s, and the literal chip eventually vanished while the metaphor — carrying a provocation around, daring someone to challenge it — stuck.