It's a flak jacket not flack. It's from a german word for anti aircraft. The jacket was first developed for bomber crews in WW2 who flew through 'flak', now used to describe the shell fragments from exploding anti aircraft artillery rounds. BTW; While there are several types of jackets, the one I wore in Vietnam had a collar that rose high on the back of the neck and which had two pockets at chest level each with a snap button. I carried a grenade in each. The jacket did not have shoulder protectors and looked more like a fising vest than a suit of armor. I did see some vests with chain mail on the shoulders ... worn by a chaplain.

Gordon