Never look a gifted horse in the mouth

"Never look a gift horse in the mouth" or "long in the tooth" are both expressions we often hear but not quite comprehend what it really means. Many may interpret is as "don't question an intelligent person" or "don't question the value of a gift". The following are two types of meaning of the phrase.

THE ETYMOLOGY MEANING:
"Don't check his age." Traditionally, horsemen use teeth as a guide to determine the horse's age. This practice is not foolproof but many factors contribute when evaluating a horse's age. Deciduous teeth (milk teeth) are grown shortly after birth. They're paler and shorter than adult teeth. They gradually shed by the age of five and are replaced by permanent teeth with a concave surface which are larger and darker in colour. At the age of ten , the Galvayne's groove begins to appear and by the age of fifteen the groove would have reached half way down the tooth. When the horse reaches its mid twenties, the Galvayne's groove begins to disappear and entirely vanish, when they are old.

IDIOM MEANING:
a) Don't try to evaluate a gift
b) be appreciative of a gift regardless whether it's big or small, instead of wanting something big and better.
c) be grateful and accept what is given.
d) don't question what is given to you, accept it as it is.