From Edward Herman & Noam Chomsky's Manufacturing Consent Pantheon 1988, p. 26:
Flak and the Enforcers: The Fourth Filter
"Flak" refers to negative responses to a media statement or program. It may take the form of letters, telegrams, phone calls, petitions, lawsuits, speeches and bills before Congress, and other modes of complaint, threat, and punitive action. It may be organized centrally or locally, or it may consist of the entirely independent actions of individuals.
If flak is produced on a large scale, or by individuals or groups with substantial resources, it can be both uncomfortable and costly to the media. Positions have to be defended within the organization and without, sometimes before legislatures and possibly even the courts. Advertisers may withdraw patronage. Television advertising is mainly of consumer goods that are readily subject to organized boycott. During the McCarthy years, many advertisers and radio and television stations were effectively coerced into quiescence and blacklisting of employees by the threats of determined Red hunters to boycott products. Advertisers are still concerned to avoid offending constituencies that might produce flak, and their demand for suitable programming is a continuing feature of the media environment. In certain kinds of fact, position, or program are thought likely to elicit flak, this prospect can be a deterrent.