If someone is too big for their boots, they behave as if they are more important or clever than they really are. Note: In the second expression, `britches' is also spelled `breeches'. I was often accused of being too big for my boots. If you ask me, he's too big for his britches since he struck it rich. Note: You can also say that someone gets too big for their boots or gets too big for their britches. In Britain, people seem to have a thing about not letting someone get too big for their boots. Getting too big for their britches, kids these days. Think the whole universe should revolve round them. Note: Britches are trousers which reach as far as your knees.