Just in case anyone was intrigued, as I was:

From Merriam-Webster:
Main Entry: flak
Pronunciation: 'flak
Function: noun
Inflected Form(s): plural flak
Etymology: German, from Fliegerabwehrkanonen, from Flieger flyer + Abwehr defense + Kanonen cannons
Date: 1938
1 : antiaircraft guns
2 : the bursting shells fired from flak
3 : CRITICISM, OPPOSITION

So - another loanword from German, then. Thanks everybody, I've learnt something today.