| Capper 1. (n.) One whose business is to make or sell caps. 2. (n.) A by-bidder; a decoy for gamblers [Slang, U. S.]. 3. (n.) An instrument for applying a percussion cap to a gun or cartridge. Thesaurus Entries agitator, agitprop, capper, catalyst, come-on man, decoy, demagogue, exciter, firebrand, fomenter, incendiary, inciter, inflamer, instigator, mischief-maker, plant, provocateur, provoker, rabble-rouser, ringleader, rouser, seditionary, seditionist, shill, stool pigeon, stoolie, troublemaker, urger |