To blow is to move air, either by exhaling through your mouth or by wind moving naturally. As a noun, a blow is a hard hit or shock.
It comes from Old English “blāwan,” meaning “to blow” or “breathe,” from a very old Germanic root. The noun sense of “strike” developed later, comparing a hit to a sudden gust.
The gentle act of blowing out birthday candles shares a word with the heavy punch of “a blow to the head.” English likes to reuse short, strong words for both physical force and emotional shocks, like “a blow to your confidence.”
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