To let air out of something; to reduce someone's confidence or pride.
From Latin 'de-' (down, away) + 'flatus' (blown), from 'flare' (to blow). The figurative sense (destroying someone's confidence) developed because it parallels the literal deflation of an inflated object losing its puffiness.
The phrase 'to deflate someone's ego' works because it borrows the physical experience of a balloon losing air—linguists call this 'conceptual metaphor,' where we understand abstract emotions through concrete physical experiences we all know.
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