Definition of capper:
(n) :
A finale; something that is conclusively better, or "caps it all off".
(n) :
A device or person that applies caps, as to bullets or bottles.
(n) :
A person that makes or sells caps.
(n) :
(slang) A person who caps i.e. is inclined to untruth, a fibber.
(n) :
(Internet slang) A person who records a television broadcast to their computer.
(n) :
(US, slang, dated) A by-bidder; a decoy for gamblers.