Idiom: Chip on your shoulder
Fig. a bad attitude that tends to get someone easily upset.“Why did you get so angry at the slightest criticism? You seem to have a chip on your shoulder.”
A perceived grievance or sense of inferiority.
The word chip has several meanings; the one that we are concerned with here is the earliest known of these, namely ‘a small piece of wood, as might be chopped, or chipped, from a larger block’. The phrase ‘a chip on one’s shoulder’ is reported as originating with the nineteenth century U.S. practice of spoiling for a fight by carrying a chip of wood on one’s shoulder, daring others to knock it off. This suggested derivation has more than the whiff of folk-etymology about it. Anyone who might be inclined to doubt that origin can take heart from an alternative theory.