Straight from the horse's mouth
What's the meaning of the phrase 'Straight from the horse's mouth'?
From the highest authority.
What's the origin of the phrase 'Straight from the horse's mouth'?
In horse racing circles tips on which horse is a likely winner circulate amongst punters. The most trusted authorities are considered to be those in closest touch with the recent form of the horse, that is, stable lads, trainers etc. The notional 'from the horse's mouth' is supposed to indicate one step better than even that inner circle, that is, the horse itself.
The phrase originated around the turn of the 20th century. The earliest printed version I can find of it is from the London newspaper Reynolds Newspaper, June 1896:
"As the great British nation takes far more interest in horse racing than in politics, the exchange of rulers would be delightful, because, look you, we'd get all our tips straight from the horse's mouths, instead of being deluded and swindled every day by their lordly owners."
The colloquial use of the expression above suggests that the paper's readers were expected to be familiar with it and earlier examples may well be found.