What is a flak and how is it used?

Answer:

For some reason the word flak was used during WWII as the name for the exploding shells fired from anti-aircraft guns. Catching some flak meant literally being under fire. Bullet proof vests are/were sometimes referred to as flak jackets, as similar types of equipment were once used to protect people from the fragments from the AA shells.

First answer by Dabac. Last edit by Dabac. Contributor trust: 486 [recommend contributor recommended]. Question popularity: 2 [recommend question].