Not the dental health, of the horse.
See also: gift, horse, look, mouth look a gift horse in the mouth.
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 dont look a given horse in the mouth, in John Heywoods.He gave his old car as a gift; I know its not a great one, but I wouldnt look a gift horse in the mouth.Meaning dont be ungrateful when you receive a gift do not be critical of a gift you receive do not refuse something good that is offered do not be unappreciative of or question a gift you have received.Here's an alternative explanation, from the question.Published by Houghton Mifflin Harcourt Publishing Company.A: "But I don't want this ancient car!" B: "I can't believe you're looking a gift horse in the mouth, especially since you can't afford to buy a car on your own!".Don't look a gift-horse in the mouth.The 16th-century English form was do not look a given horse in the mouth.Never look a gift horse in the mouth. .Why shouldnt you look in its mouth? .If I were you, I wouldnt look a gift horse in the mouth.(Usually with a negative.) Never look a gift horse in the mouth.
Up vote 37 down vote, a horses teeth are regarded as a good guide to its age.
When you buy a horse you check its teeth to see if they match the age of the horse according to the seller.
Don't question the value of a gift.
Why shouldn't you look in its mouth?Same goes for the horse, you didn't have a horse to begin with so giving one unto you appreciate it and nevermind the flaw of age.Where does it come from?This can be applied as an analogy to any gift: Don't inspect it to make sure it matches some standard martin miller gift set you have, just be grateful!He was employed at the courts of Henry viii and Mary I as a singer, musician, and playwright.When given a horse, it would be bad manners to inspect the horse's mouth to see if it has bad teeth.However, the first print occurrence in English is found in 1546 in John Heywoods A dialogue conteinyng the nomber in effect of all the prouerbes in the Englishe tongue (middle English).Up vote 2 down vote, it is rude to be critical of a gift.