1 general:: Also, too big for one's boots. Conceited, self-important, as in Ever since he won that tournament he's gotten too big for his britches, or There's no talking to Jill anymore? she's just too big for her boots. This metaphoric idiom alludes to becoming so "swollen" with conceit that one's pants or boots no longer fit. [Late 1800s]
American Heritage Idioms