Flak is an abbreviation for a German anti-aircraft (Fliegerabwehrkanone) that dawned in popularity during WWII. It was launched from the ground toward invading enemy aircraft and set to explode at a certain altitude. When the flak detonated, shrapnel flung in random directions, destroying aircraft and airmen.
In order to diminish the effect the shrapnel had on the airmen, the British invented the flak jacket, an early form of body armor. It was essentially pieces of metal sewn into ballistic nylon, which ended up being very heavy (22 pounds). They were too cumbersome for the small bomber planes the British flew in WWII, so they offered their jackets to the Americans. The United States Army Air Forces quickly adopted them into their military standard, and the Navy soon utilized them as well. Both branches of the military found that the flak jackets proved to be very useful and saved many lives.
Flak jackets continued to be popular throughout the Vietnam War as well, although by then significant improvements had been made regarding the lightness of the metal.
Flak jackets eventually morphed into today’s body armor and have been spread throughout the military and even local law enforcement. Find more stories about flak in Fold3′s WWII Missing Air Crew Reports.