too big for britches
too big for (one's) britches
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 britches ever since he got that promotion. John's been too big for his britches now that he's been scouted by pro teams.
too big for one's britches
Rur. too haughty for one's status or age. Bill's getting a little too big for his britches, and somebody's going to straighten him out. You're too big for your britches, young man! You had better be more respectful.
too big for (one's) britches
Overconfident; cocky.