Never look a gift horse in the mouth. What is a gift-horse? Why shouldn’t you look in its mouth? What does this idiom actually mean?
A gift horse is a horse that was a gift, quite simply. When given a horse, it would be bad manners to inspect the horse’s mouth to see if it has bad teeth. This can be applied as an analogy to any gift: Don’t inspect it to make sure it matches some standard you have, just be grateful and say thank you!
Where does it come from?
As with most proverbs the origin is ancient and unknown. We have some clues with this one however. The phrase appears in print in English in 1546, as “don’t look a given horse in the mouth”, in John Heywood’s A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue.