The expression or proverb “to look a gift horse in the mouth” is so old that its origin cannot be determined.
It has been traced to the writings of St. Jerome, one of the Latin Fathers of the fourth century, who then labeled it a common proverb.
The expression, or a variant proverb, occurs in French, Italian, Spanish, and other languages of Europe. The reference is, of course, to the bad manners displayed by one who receives a gift if he examines it for defects.
Up to a certain age, the age of a horse can be determined by looking at its teeth; though it may appear to be young and frisky, the number or condition of the teeth may show it to be almost fit for nothing but the glue-works.