ear to the ground, to have/keep an

To be well informed. The allusion here, one writer conjectures, is to the days of cowboys and Indians, when one literally put one’s ear to the ground in order to hear the sound of horses miles away. An Americanism dating from the late nineteenth century, the term was a cliché by the time Stanley Walker poked fun at it (and two others) in The Uncanny Knacks of Mr. Doherty (1941): “He had his ear to the ground and his eye on the ball while they were sitting on the fence.”