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have a swelled head
To be conceited, arrogant, or self-aggrandizing. Cheryl has such a big head on her, all she ever talks about is herself. Janice has had a bit of a big head ever since she got that promotion.
Fig. a state of being conceited. (Typically: get ~; have ~; give someone ~.) John got a swelled head after he won the prize. Don't get a swelled head from all this success.