too big for your boots
too big for (one's) boots
Overconfident in one's importance, skill, or authority; behaving as if one is more important or influential than one actually is. He's gotten too big for his boots ever since he got that promotion. John's been too big for his boots now that he's been scouted by pro teams.
too big for your bootsBRITISH, INFORMAL or
too big for your britchesmainly AUSTRALIAN, INFORMAL
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.