Wordorigins.org

Whistleblower a word for an employee, especially a civil servant, who publicly denounces illegal or wasteful practices, comes from the phrase blow the whistle. P.G. Wodehouse uses the phrase in 1934’s Right Ho, Jeeves:

Now that the whistle had been blown on his speech, it seemed to me that there was no longer any need for the strategic retreat which I had been planning.

And Raymond Chandler picks up the term on the other side of the pond in 1954’s The Long Good-Bye:

Come on, Marlowe. I’m blowing the whistle on you.

The metaphor behind the phrase is somewhat obscure. It could be that of a policeman blowing his whistle to stop an illegal activity or of a referee using a whistle to call a foul.

The term whistleblower, itself, dates to at least 1958, when it appears in the Mansfield News-Journal (Ohio) of 10 October:

The whistleblower on that $50,000 a month call-girl story was a witch, who tried to tap Bea Garfield, alleged madam, for $250.

(Sources: Oxford English Dictionary, 2nd Edition; Oxford Dictionary of American Political Slang)