The Collaborative International Dictionary of English v.0.48:
Capper \Cap"per\, n. 1. One whose business is to make or sell caps. [1913 Webster] 2. A by-bidder; a decoy for gamblers. [Slang, U. S.] [1913 Webster] 3. An instrument for applying a percussion cap to a gun or cartridge. [1913 Webster]