1
general::
From a reliable source, on the best authority. For example, I have it from the horse's mouth that he plans to retire next month. Also put as straight from the horse's mouth, this expression alludes to examining a horse's teeth to determine its age and hence its worth. [1920s]
American Heritage Idioms
2
general::
Phrase(s): (straight) from the horse’s mouth
Fig. from an authoritative or dependable source. (See also get something straight from the horse’s mouth.) • I know it’s true! I heard it straight from the horse’s mouth! • This comes straight from the horse’s mouth, so it has to be believed.
McGrawhill's American Idioms And Phrasal Verbs