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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.