Definition of too big for one's britches (or breeches) in US English:

too big for one's britches (or breeches)

phrase

informal
  • Conceited.

    • ‘If you've got wealth, privilege or exalted connections crowned with success, then in this country you are judged way too big for your boots and in need of an urgent reality check.’
    • ‘He's been getting far too big for his boots recently.’
    • ‘They long ago became just too big for their boots.’
    • ‘She has no airs and graces and will put you in check if she thinks you're being too big for your boots - she's done it to me before.’
    • ‘Basically, they think you're too big for your boots and want you cut down to size.’
    • ‘There are plenty of applicants to fill the space left by a boy band who got too big for their boots.’
    • ‘After his eldest boy's graduation ceremony he said: ‘Don't get too big for your boots, son.’’
    • ‘I don't know how I'm going to get paid, but I'd rather go out into the brave new world than live with dinosaurs that are far too big for their boots.’
    • ‘‘He told me never to be too big for your boots and always be friendly to everyone,’ she said.’
    • ‘I think that room is appearing again for the little man and people want to say to the big guys that they're getting too big for their boots.’
    conceited, full of oneself, cocky, boastful, arrogant, cocksure, above oneself, self-important, immodest, swaggering, strutting
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