What does "a chip on your shoulder" mean — and why is it funny?

informal

Meaning

A lasting feeling of resentment or grievance, often making someone quick to take offence.

Where it comes from

From an American custom of the 1800s: a boy spoiling for a fight would balance a wood chip on his shoulder and dare another to knock it off.

Why it is funny

The mild humor is the literal silliness of the original ritual. A serious, simmering grudge began life as a small piece of wood, perched on a shoulder, waiting for trouble.

Used in a sentence

"He's had a chip on his shoulder ever since he was passed over for the job."