The Horse’s Mouth??

From www.grammars.com.

Straight from the horse’s mouth is an idiom that is linked to the sporting world. An idiom is a word, group of words or phrase that has a figurative meaning that is not easily deduced from its literal meaning. We will examine the definition of the phrase straight from the horse’s mouth, where it came from and some examples of its use in sentences.

Straight from the horse’s mouth describes information that has been received directly from a source of authority and has not been interpreted or diluted by a middleman. It is a source that may be trusted implicitly. The idiom straight from the horse’s mouth seems to have been coined around the turn of the twentieth century, used at first in horse racing circles. The idea is that a racing tip has been received from the race horse himself. Some believe that the phrase relates to checking the teeth of a horse in order to see what sort of physical shape the horse is in, and whether he is capable of winning a race.

Today, the idiom is used in a wide variety of situations to mean learning information from an impeccable source. Note that the word horse’s is a possessive noun, and therefore requires an apostrophe before the s.