The noun dog whistle designates a high-frequency whistle producing a sound audible to dogs but not to humans.

This noun is also used figuratively, in politics, of a statement or expression which in addition to its ostensible meaning has a further, typically less publicly acceptable interpretation or connotation intended to be understood only by a specific target audience.

The following explanation of this political use of the noun dog whistle is from High pitch, low politics: Dog-whistle politics can take you only so far, published in The Economist (London, England) of Monday 23^rd May 2005:

Over the past few weeks, a new expression has entered the Westminster lexicon: dog-whistle politics. It means putting out a message that, like a high-pitched dog-whistle, is only fully audible to those at whom it is directly aimed. The intention is to make potential supporters sit up and take notice while avoiding offending those to whom the message will not appeal.

In political contexts, the noun dog whistle is often used attributively—and hyphenated—as in the following title of an article by Ben Quinn, political correspondent for The Guardian, published in The Guardian (London and Manchester, England) of Tuesday 26^th September 2023:

LGBT+ Conservatives patron accuses Braverman of ‘dog-whistle’ politics
UK home secretary will say in US that Britain should not grant asylum to people who simply express fear of discrimination for being gay

Before the noun dog whistle came to be used in politics, it was already used figuratively of a message, signal, etc., that only a category of persons—or one person—can understand. The following are three examples of this figurative use:

1-: From the review of The Garden of Earthly Delights (London: HarperCollins Publishers, 1993), by Nicholas Salaman—review by Peter Matthews, published in the Observer (London, England) of Sunday 1^st August 1993:

Either this book is sophomoric and stupid, or it’s like a literary dog whistle, too clever to be perceived by the ordinary critical faculties. You decide.

2-: From an account of the press conference held on Tuesday 18^th January 1994 by Bobby Ray Inman (born 1931), a retired United States Navy admiral who held several influential positions in the United States Intelligence Community—account by Martin Schram, published in Newsday (New York City, New York, USA) of Sunday 23^rd January 1994:

Inman claimed that he knew from “reports” (dog-whistle reports that no one else seems to have heard) that Dole and Safire had conspired to get him.

3-: From Brash and Buzz-Cut Atop Her Beloved Tank, a review of the 1995 U.S. film Tank Girl, directed by Rachel Talalay—review by Janet Maslin, published in The New York Times (New York City, New York, USA) of Friday 31^st March 1995:

“Tank Girl” is a comic-based movie with the flashy pop style of a real comic book, but that’s a mixed blessing. It’s a comic book that takes an hour and three-quarters to read, for one thing. And its storytelling isn’t moved forward by anything better than an occasional need to turn the page.
But “Tank Girl” may be sending out a dog-whistle signal to its own special constituency, fans of an androgynous punk heroine who’s so underground she’s almost buried.

The first two political uses of the noun dog whistle that I have found are from articles by Jim Coyle, published in The Ottawa Citizen (Ottawa, Ontario, Canada):

1-: Of Sunday 15^th October 1995—article titled Common sense not at all common in Harris government decision making:

One of the acts of political genius of the new Ontario government has been to set the language of debate. Its actions and supporters are invariably portrayed as the epitome of common sense; its critics are the grasping voices of special interests. On the lips of Premier Mike Harris, the term “special interest” has the tone of epithet. It’s an all-purpose dog-whistle that those fed up with feminists, minorities, the undeserving poor hear loud and clear.

2-: Of Sunday 3^rd November 1996—article titled Harris comments play into racist hands:

The premier stepped into a corridor to take questions on the protest just concluded in the streets of Toronto against his government. Harris shrugged.
“They are a lot of special-interest groups, if you saw the banners go by from some of the Communist parties, as I saw, and an Iraqi group and an Iranian group…”
Pens scribbled. Eyes popped. For it was curious that he would choose to single out what could only have been a handful of people from tens of thousands.
It would be nice to think the premier was merely being thoughtless, rather than calculating, that he was not blowing on the dog whistle that only racists hear. But the comment made no point, seemed a propos of nothing, other than the gratuitous highlighting of ethnicity.

This political use of the noun dog whistle has given rise to the verb dog-whistle, which means: to use the political strategy consisting in targeting a potentially controversial message to specific voters while avoiding offending those voters with whom the message will not be popular.
—Hence the verbal noun dog-whistling, which designates the use of this kind of political strategy, and the noun dog-whistler, which designates a politician who uses this kind of strategy.

The earliest occurrences of these words that I have found are from The Australian Jewish News (Sydney, New South Wales, Australia) of Friday 26^th September 1997:

Kernot condemns Howard’s “dog-whistling”

Vic Alhadeff
Dog-whistling is a carefully-crafted way of speaking in which the words seem innocuous, yet give a clear message, Senator Cheryl Kernot said last week.
“Both Pauline Hanson and Prime Minister John Howard are good dog-whistlers, giving out a message to which racists respond—such as in their assault on political correctness, multiculturalism, native title and immigration.”
“The difference, however, is that he is leading the nation, and he has a responsibility to lead it away from a flirtation with racism. You don’t get the mantle of leadership to follow opinion polls. Leadership means taking risks and putting the nation first. I hope the Prime Minister doesn’t fail the Australians of this generation.”
The leader of the Australian Democrats was addressing the NSW Jewish Board of Deputies plenum.