Example: “I heard he’s going to go to graduate school.” “It’s true! I heard it straight from the horse’s mouth.”

Definition: From the source

Explanation: The phrase originated in horse racing circles. Before people would bet on horses, they would ask the people closest to the horses, like the trainers and riders, for their opinions about the race. Over time, ‘straight from the horse’s mouth’ began to mean straight from the most trusted and knowledgeable source.