Manufacturing Consent by Herman and Chomsky, 1988, Excerpts
“Flak” refers to negative responses to a media statement or program. If flak is produced on a large scale 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 courts. Advertisers may withdraw patronage. Television advertising is mainly of consumer goods that are readily subject to organized boycott.
The ability to produce flak that is costly and threatening is related to power. Flak from the powerful can be either direct or indirect. The direct would include letters or phone calls from the White House or from the FCC to the television networks asking for documents used in putting together a program, or from irate corporate sponsors asking for reply time or threatening retaliation.