A flack is a person—especially a press agent or publicist—who is capable of turning any criticism into an advantage for his or her employer. Flak (usually a mass noun) means (1) antiaircraft artillery, and (2) excessive or abusive criticism (the second definition derives metaphorically from the first).
Examples
Writers commonly use flack when they mean flak in its second sense—for example:
FIJI water has gotten so much flack lately, I’m starting to feel sorry for them. [Triple Pundit]
These writers use flak well:
RIM has taken a lot of flak from CNET and others for its decision to pair the BlackBerry PlayBook tablet with a BlackBerry smartphone … [CNET]
Brown budget plan draws flak from right, left [LA Times]
And these writers use flack well:
He promptly became a political flack. [Gawker]
So far, Carney’s been successful in making the transition from journalist to administration flack. [Yahoo! News]
For some reason, the verb cop frequently appears with flak, creating constructions like these:
Palin cops flak for term offensive to Jews [Sydney Morning Herald]
But back to Nicky Watson, that interview, and that question, for which he copped considerable flak, accused of being “pervy”. [The Southland Times]
This appears to be a mostly Australian/New Zealand thing. Can anyone explain it?

