Q: Does the expression “straight from the horse’s mouth” have anything to do with actual horses?
A: The expression, which means reliable or on good authority, has two possible origins. The most likely is that it comes from horse-racing circles: a tipster supposedly has inside information so good that it comes straight from the horse. According to the Random House Historical Dictionary of American Slang, the expression goes back to 1917.
The second possibility is that the phrase comes from the world of horse-trading, but I can find no good evidence to support it. According to this explanation, a smart buyer examines a horse’s teeth to determine its age and general health, so reliable information about the animal comes from its mouth. This, by the way, is the origin of the expression “don’t look a gift horse in the mouth” (don’t quibble about something you aren’t paying for).