How did 客气 come to mean "polite"?
客 - guest
氣 - air. spirit ...
客氣 - in the spirit of a guest
When you're a guest in someone's house, you behave like a guest should, that is, be polite.
When you're a guest, your host might say “不要客氣” （don't behave like a guest). This is not to say you shouldn't be polite. It is rather their way of saying &quot;make yourself at home&quot;.
Another way to translate 不要客氣 is &quot;don't stand on ceremony&quot;. 客氣 encompasses both of these ideas - politeness and formality. Both are positive attributes, but too much of either tends to create distance. Being the right degree of 客氣 is truly an art.