Information received directly from the most reliable or authoritative source, typically the person most closely involved or knowledgeable about a situation.
This phrase comes from horse racing and trading, where the most reliable way to determine a horse's age and condition was to examine its teeth directly. The practice of 'looking a gift horse in the mouth' was related but opposite - this phrase emphasizes getting information from the primary source rather than relying on secondhand accounts.
The horse's mouth was literally the most reliable source of information about the animal's value and health, making this a perfect metaphor for authoritative information. Interestingly, horses' teeth continue growing throughout their lives and wear in predictable patterns, so experienced traders could determine age quite accurately - making the horse's mouth a genuinely trustworthy source of data.
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