too big for your britches
US /tuː bɪɡ fɔːr jʊər ˈbrɪtʃɪz/
UK /tuː bɪɡ fɔːr jʊər ˈbrɪtʃɪz/
1.
acting in a way that shows you think you are more important or capable than you really are; arrogant or conceited
:
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Ever since he got that promotion, he's been acting too big for his britches.
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Don't get too big for your britches just because you won one game.