Origin of Phrases - S
Straight from the horse's mouth
Meaning: Directly from the source.
Example: If you want the real story you have to get it straight from the horses mouth.
Origin: This is a boast of confidence from a racetrack tipster, who says he gets his information from the horses themselves—thereby assuring the bettor that the info is the correct.
Alternatively, A horse trader would bend the ear of a prospective buyer with all kinds of talk about the animal, but for a clear measure of its worth, one can simply look in the animal's mouth. You can tell a great deal about a horse from its mouth. Age, nutrition, general health of the horse, and if it had been over reined.
If a horse is unruly you have to rein it in a lot, and this shows in the horse's mouth.