Straight from the horse's mouth

Origin

In the vibrant, often deceitful world of horse racing, a punter’s most coveted possession was an insider tip—information directly from the stable. While bookmakers and touts spun tales, the truest, most undeniable facts about a horse's age and condition could only be found by literally examining its teeth, a direct inspection of the "horse's mouth." This precise, unquestionable method of determining a horse’s value soon became a vivid metaphor. To hear something "straight from the horse's mouth" meant getting information from the ultimate, unimpeachable source, often a jockey or trainer, ensuring its accuracy and leaving no room for doubt or second-guessing.