Not to look at gift horse in the mouth

Question

I understand the meaning of the saying and the context it’s used in.

What I don’t understand is why you would look a horse (or gift horse) in the MOUTH. What is special about its mouth?

In: Other

Answer 1

Horses teeth continuously wear down because of their food. So you can approximate a horses age by looking at its teeth.

You do this when horse trading to validate the claim of how old the horse is.

But if its a gift horse, it is rude to go “Oh, ima check how old it really is”