From Longman Dictionary of Contemporary Englishflak jacketˈflak ˌjacket noun [countable] PMa special coat made of strong heavy material to protect soldiers and policemen from bullets
Examples from the Corpus
flak jacket• If that happens, Freedman is going to have to get a flak jacket and write another book.• Strapped into an armor-lined cockpit and trussed up in a flak jacket and helmet, Air Force Capt.• McVeigh wore a purple shirt with a flak jacket beneath it and brown slacks.• Tucking himself into a ball of boots and flak jacket, the Squat bounced and rolled half a dozen times.• Primo notices his dark swollen belly, pushing out between the flaps of a green flak jacket. 