The Collaborative International Dictionary of English v.0.48:

Racer \Ra"cer\ (r[=a]"s[~e]r), n.
   1. One who, or that which, races, or contends in a race;
      esp., a race horse.
      [1913 Webster]

            And bade the nimblest racer seize the prize. --Pope.
      [1913 Webster]

   2. (Zool.) The common American black snake.
      [1913 Webster]

   3. (Mil.) One of the circular iron or steel rails on which
      the chassis of a heavy gun is turned.
      [1913 Webster]

WordNet (r) 3.0 (2006):

racer
    n 1: someone who drives racing cars at high speeds [syn:
         racer, race driver, automobile driver]
    2: a fast car that competes in races [syn: racer, race car,
       racing car]
    3: an animal that races
    4: slender fast-moving North American snakes