From the original or most reliable source. A: "Is our test really getting rescheduled?" B: "Yep, our teacher was ahead of me in the lunch line, so I heard it straight from the horse's mouth."
straight from the horse’s mouth
mod. directly from the source, not just a horse. (As if a racehorse were giving racing tips.) This came straight from the horse’s mouth. It’s Zeerocks Copy in the sixth race.