the horse's mouth
A first-hand or extremely reliable source (of some piece of information). A: "Is our test really getting rescheduled?" B: "Yep, our teacher was ahead of me in the lunch line, so I heard it straight from the horse's mouth." I have a source who works directly for the president, so this tip-off is coming straight from the horse's mouth.
the horse's mouth
A source of information regarded as original or unimpeachable.
the horse's mouth
The source of a truthful or honest statement. A likely origin of the phrase is how a horse's age can be determined with a fair degree of accuracy by the number, length, and shape of its teeth. A seller might exaggerate an animal's age, but the merchandise's mouth would reveal the truth.