Getting information from the highest authority, unadulterated by an intermediary.
Tips on which horse is a likely winner circulate amongst horse racing enthusiasts. The most trusted authorities are those who spend a lot of time with the horses. The phrase ‘from the horse’s mouth’ is supposed to be one step better than even that inner circle of humans, that is, the horse itself.
The expression came into racetrack use about 1830 and was part of everyday speech by 1900. The earliest printed version was in the Syracuse Herald, May 1913, and clearly indicates the horse racing context: “I got a tip yesterday, and if it wasn’t straight from the horse’s mouth it was jolly well the next thing to it.”
A horse’s teeth reveals the horse’s age and the health. A smart horse trader or buyer could get all the information he needed from ‘the horse’s mouth.’ This fact has been known for centuries and gave rise to the adage, ‘Don’t look a gift horse in the mouth,’ dating to the fifth century.
The English language uses many terms and phrases which harken back to when horses were of extreme importance, and contain useful information about equine behavior or the care and treatment of horses. Phrases like ‘stubborn as a mule,’ ‘beating a dead horse,’ ‘horseplay,’ ‘horsing around’ and ‘horse laugh’ hardly need an explanation.
A related phrase is, ‘If you don’t hear it from the horse’s mouth, you’re hearing it from a horse’s ass.’ These words form part of a song by the Gaddabouts. It appears to be trade-marked and appears on mugs and tees advertised on the Web.
The Horse’s Mouth was the title of a novel by Joyce Cary (1944).