too big for your britches
US /tuː bɪɡ fɔːr jʊər ˈbrɪtʃɪz/
UK /tuː bɪɡ fɔːr jʊər ˈbrɪtʃɪz/
1.
自命不凡, 自高自大, 狂妄自大
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.
自从他得到那次升职后，他就开始自命不凡了。
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Don't get too big for your britches just because you won one game.
不要因为赢了一场比赛就自命不凡。