too big for one's breeches

too big for one's breeches or 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.
Categories: adjective feelings
Source: A Dictionary of American Idioms

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