swim \swim\, n.
   1. the act of swimming; a gliding motion, like that of one
      swimming. --b. jonson.
      1913 webster

   2. the sound, or air bladder, of a fish.
      1913 webster

   3. a part of a stream much frequented by fish. eng.
      1913 webster

   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
see also:
swim bladder to be in the swim