Although what became known as a "silent dog whistle" was invented by Francis Galton in 1876, the term silent dog whistle first arises in Google Books search results in the 1930s. From A.M. Low, Popular Scientific Recreations (1933):
A "Silent" Dog Whistle.—Animals, of course, can hear higher notes than we can, and with a little ingenuity it is possible to make a whistle that cannot be heard by human beings, but to which a dog answers readily.
Mordecai Siegal, The Good Dog Book: Loving Care (1977) offers this discussion:
Dogs can hear better than humans but not quite as well as cats. They can hear sounds that we cannot and can listen for a familiar footstep long before it comes within human earshot. The so-called Galton whistle or silent dog whistle is not silent at all. It is pitched at a frequency too high for the human ear but quite clear to the canine or feline ear.
The fullest discussion of dog whistle in the context of politics appears in William Safire, Safire's Political Dictionary (2008). Here are some extracts from Safire's coverage:
dog whistle politics The use of messages embedded in speeches that seem innocent to a general audience but resonate with a specific public attuned to receive them.
Commenting on the President's historical reference [in 2004 to the Supreme Court's 1857 Dred Scott decision, which was overturned eight years later by the thirteenth amendment], David D. Kilpatrick wrote in The New York Times: "The potential double meaning rekindled speculation among Mr. Bush's critics that he communicates with his conservative Christian base with a dog whistle of code words and symbols, deliberately incomprehensible to secular liberals." The message would be pitched high enough to rally the whistler's core constituency without unduly arousing the opposition.
The political dog whistle may have derived from the term's use by pollsters. Richard Morin, director of polling for The Washington Post, observed in a 1988 article that "Subtle changes in question-wording sometimes produce remarkably different results.... researchers call this the 'Dog Whistle Effect': Respondents hear something in the question that researchers do not."
The effect has been noticed in other fields. In 1989, Russell Smith noted in the Dallas Morning News that an MTV spot "which has no obvious reason to exist, is like a secret signal, a dog whistle blown on thirtysomething frequency. Come back, MTV beckons, in a language it hasn't spoken in years." Two years earlier, in New York magazine, Geraldine Stutz, three-decade president of Henri Bendel in New York City, described her taste in "Dog-whistle fashion," meaning that she specialized in stocking in her store "clothes with a pitch so high and special that only the thinnest and most sophisticated women would hear their call."
Yet another area to which "dog whistle" has been applied is men's emotional response to music. From Brett Mandell, Is This Heaven? The Magic of the Field of Dreams (2002):
As for why the film resonated so well—especially for men—[Tim] Busfield has a theory involving the film's score. According to Busfield, "There's a note that James Horner hits in his music about the time when Kevin's character says to his dad, 'You wanna have a catch?' There's a note that he hits that's like the dog whistle for men's emotions. It sets off something that men, the emotional side of men, just lets go."
The earliest use of the term in a political sense that a Google Books search finds is from Laurie Oakes, "A Sly-Dog Race Card," in The [Sydney, New South Wales] Bulletin (November 13, 2001):
The key feature of this election campaign has been a clever use of what professionals call "dog whistle politics". A dog whistle is pitched so high that dogs hear it but humans do not. Dog whistle politics involves pitching a message to a particular group of voters that other voters do not hear.
Whether use of dog whistle in a strictly political sense goes back much further than that is hard to judge. One early but ambiguous instance of the term in a political context appears in "North's 'Talk' Muzzled," a very brief letter to the editor of The [Boston, Massachusetts] Heights: The Student Weekly of Boston College (October 24, 1988) by Raymond McNally, a professor of history:
As Oliver North comes to BC muzzled by his lawyers, since he is under indictment, going to hear him "talk" is like listening to a bird dog whistle.
Ultimately, it is unclear whether Professor McNally's point was that North's speech wouldn't be worth hearing because it would be (metaphorically speaking) pitched beyond the hearing of human listeners in order to minimize North's legal liability for anything he might say, or whether he was implying that only listeners attuned to North's dog-whistle language would be in a position to grasp the true significance of his remarks. The former possibility strikes me as being rather more likely.
I would be remiss if I didn't acknowledge the astounding discovery by Dr. Rafik Tajerian of Budapest, Hungary, that a specially (albeit accidentally) modified silent dog whistle "can stifle a grown man's wandering eye." You can read all the details—including a Before and After dramatic photo reenactment of the whistle's effectiveness—in "Hold It, Mister! Ultrasonic Whistle Stops a Cheatin' Man in His Tracks," in Weekly World News (January 22, 2002). The report of this scientific breakthrough undoubtedly would have earned above-the-fold front-page billing if not for the competition from a shocking historical exposé, "Abraham Lincoln Was a Woman!"
Aside from the significant philosophical and practical overlap between dogs, politicians, and cheatin' men, this story has nothing to do with "political dog whistles"—but I strive to be fair and balanced in my presentation of all the crucial information that I encounter in my research.