A "flak jacket" is a protective vest that shields someone from bullets and shrapnel. It is worn by soldiers or individuals in dangerous situations to keep them safe. Full definition
And then, when they got near the end, someone came over and put what felt like an 80 - pound flak jacket on me, handed me an M16 rifle, and said, «OK, you're up.» (inc.com)
The campaign rolled out its first television spot, featuring jarring images of schoolchildren in flak jackets and combat helmets, to illustrate Ryan's support for an assault weapons ban, this week. (hudsonvalleyone.com)
As Frank — and, by extension, the movie — puts it, the scam they're pulling isn't much worse that the deceptive theater of the big networks, where reporters don flak jackets to broadcast «ground - level» reports from an alley behind a hotel in Quito. (film.avclub.com)