The Collaborative International Dictionary of English v.0.48:

Swim \Swim\, n.
   1. The act of swimming; a gliding motion, like that of one
      swimming. --B. Jonson.
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   2. The sound, or air bladder, of a fish.
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   3. A part of a stream much frequented by fish. [Eng.]
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   Swim bladder, an air bladder of a fish.

   To be in the swim, to be in a favored position; to be
      associated with others in active affairs. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

swim bladder
    n 1: an air-filled sac near the spinal column in many fishes
         that helps maintain buoyancy [syn: air bladder, swim
         bladder, float]