Flack or flak?

Although it seems to be gaining acceptance as an alternative spelling of flak, the obscure "flack" originally meant huckster, press agent, publicist or some other form of spin doctor.

In contrast, the more commonly used (or at least intended to be used) "flak" means military shells or anti-aircraft artillery (perhaps from WWII German anti-aircraft guns known as Fliegerabwehrkanone), but has taken on a metaphorical secondary meaning of excessive or abusive criticism.

(Reference: http://www.word-detective.com/2009/07/19/flack-flak/)