too big for one's breechesto feel more important than one really is, to think that you are more important than you really are
[TahlilGaran] Dictionary of Contemporary English ▲
too big for one's breeches adj. phr. Too sure of your own importance; feeling more important than you really are.
That boy had grown too big for his breeches. I'll have to put him back in his place.
When the teacher made Bob a monitor, he got too big for his boots and she had to warn him. [TahlilGaran] English Idioms Dictionary ▲