"A method of persuasion in which damaging rumors or innuendo are spread about the
target, while the source of the rumors seeks to avoid being detected while spreading them (for example, a political campaign might distribute
anonymous flyers attacking the other candidate). It is generally considered unethical in open societies, particularly in matters of public policy. The
speed and anonymity of communication
made possible by modern technologies like the Internet has increased public awareness of whispering campaigns and their ability to succeed. This phenomenon has also led to the failure of whispering campaigns, as those seeking to prevent them are able to publicize their existence much more readily than in the past. Whispering campaigns are defended in some circles as an efficient mechanism for underdogs to attack the powerful while lacking resources."
Source:
Wikipedia.org