dum-dum
 n.— «When he returned in ’03, McLain was equipped with a thick cushion to wear atop his helmet (he refers to it as a “dum-dum”), which softened any potentially dangerous blows to the head.» —“McLain Making Progress” by Mitch Dobbs ‘Bama (Al.) June 24, 2004. (source: Double-Tongued Dictionary)