Keep in mind that before the invention of the automobile (and even for decades after), horses were extremely more prevalent than they are today. People bought and sold horses in immense numbers. And the health of the horse was vital as they were too expensive to be considered “expendable”. So everyone knew to check the health of the horse before purchasing one.
However, if you were “gifted” a horse, it would be kind of rude to check the health of the horse in front of the person giving it to you. Hence the phrase “Never look a gift horse in the mouth” was implying that if someone gifts you something, be polite.
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