The Collaborative International Dictionary
To be in the swim
Swim \Swim\, n.
The act of swimming; a gliding motion, like that of one swimming.
--B. Jonson.The sound, or air bladder, of a fish.
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A part of a stream much frequented by fish. [Eng.]
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.]