= to hear information directly from the source being talked about or highest authority
Origin: When people are betting on horse races, they want to get tips (information) about what horses to bet on by talking to the people who are closest to the horses like the jockey, horse trainer etc.
Ex: I don’t believe what people are saying she said. I going to go ask her and hear it “straight from the horse’s mouth.”
Ex: I heard John quit this morning. I can confirm it’s true because I just had lunch with John, and heard it “straight from the horse’s mouth.”