The phrase “Never Look a Gift Horse in the Mouth”, generally means don’t be ungrateful when you receive a gift. The proverb, it seems, can be traced back to St. Jerome who referred to it as a common saying in his introductory remarks to the Epistle to the Ephesians in his translation of the New Testament: “Equi donati dentes non inspiciuntur” translating loosely into “A given horse’s teeth are not inspected.”
The first known printing appears in English writer John Heywood‘s “A Dialogue Conteinyng the Nomber in Effect of All the Proverbes in the Englishe Tongue”, where it is written as “No man ought to looke a geuen hors in the mouth.”
Horses’ gums recede as they age making the teeth appear to grow long (hence the term, “long in the tooth“). Therefore, inspecting the teeth of a horse given as a gift would mean that recipient is trying to see if the horse is old (undesirable) or young (more desirable).