If you have a Chip On Your Shoulder then you’re looking for an argument for no apparent reason, at least not one that is obvious to anyone else. Its origin lies 200 years ago, in a custom used by American schoolboys to challenge each other to a fight. One would place a twig or piece of bark – a chip of wood – on his shoulder and challenge another boy to knock it off. If he did so then a fight ensued. Then, as now, the phrase is used to describe someone who is spoiling for a row.