Flak, not flack.
A flack is a PR person.
Flak is an abbreviation of the German Fliegerabwehrkanonen, from Flieger flyer + Abwehr defense + Kanonen cannons. Of which the 88 was perhaps the most famous, used in a dual role as an antitank weapon.
https://www.americanheritagemuseum.org/jml/german-8-8cm-flak-36-anti-everything-gun/
hehe thanks