Question:

If you're getting shot at by antiaircraft guns, or receiving unfriendly criticism, are you taking flak or flack?

Answer:

flak

How to Remember It:

Although flack is an established variant, the more foreign-looking flak is the original spelling and the better choice. Flak was originally a German acronym for Fliegerabwehrkanonen – from FLieger ("flyer") + Abwehr ("defense") + Kanonen ("cannons") – which basically means "antiaircraft gun."

That use of flak in English dates back to 1938. In the decades after the war it took on its civilian meaning of "criticism."

(A flack, meanwhile, is a PR agent or someone who provides publicity.)