The Collaborative International Dictionary of English v.0.48:

Ringer \Ring"er\, n.
   1. One who, or that which, rings; especially, one who rings
      chimes on bells.
      [1913 Webster]

   2. (Mining) A crowbar. --Simmonds.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ringer \Ring"er\, n. (Horse Racing)
   A horse that is not entitled to take part in a race, but is
   fraudulently got into it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ringer
    n 1: a person who rings church bells (as for summoning the
         congregation) [syn: toller, bell ringer, ringer]
    2: a person who is almost identical to another [syn: ringer,
       dead ringer, clone]
    3: a contestant entered in a competition under false pretenses
    4: (horseshoes) the successful throw of a horseshoe or quoit so
       as to encircle a stake or peg