too big for one's boots
CONCEITED, full of oneself, swollen headed, cocky, arrogant, cocksure, above oneself, self important, vain, self satisfied, pleased with oneself, smug, complacent, big headed, vainglorious

[TahlilGaran] Dictionary of Contemporary English ▲

too big for boots
too big for (your) boots
[British, American & Australian, informal, American, informal]
someone who is too big for their boots behaves as if they are more important or more clever than they really are.
Since he was made team captain, he's been ordering us all around and generally getting much too big for his boots.

[TahlilGaran] English Idioms Dictionary ▲

too big for one's boots
• too big for one's boots/breeches
to feel more important than one really is; to think that you are more important than you really are
Our manager is too big for his breeches and needs someone to make him realize that he is not so important.

Our new boss is too big for his britches and needs to change his behavior.

[TahlilGaran] English Idioms Dictionary ▲

too big for one's boots
{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 ▲