When we hear someone say he had such and such a piece of information “straight from the horse’s mouth,” we know that he means that he received it from the highest authority, from the one person whose testimony is beyond question.
The expression comes from horse racing and has to do with the age of the racers.
Scientists tell us that the most certain evidence of the age of a horse is by examination of its teeth, especially those of the lower jaw. The first of its permanent teeth, those in the center of the jaw, do not begin to appear until the animal is two and a half years old.
A year later the second pair, those alongside the first, begin to come through, and when the animal is between its fourth and fifth year, the third pair appears,
Thus, no matter what an owner may say of the horse’s age, by an examination of its lower jaw an experienced person can get his information at first hand, straight from the horse’s mouth.