Source : Webster's Revised Unabridged Dictionary (1913)

Flipper \Flip"per\, n. [Cf. {Flip}, {Flippant}.]
   1. (Zo["o]l.) A broad flat limb used for swimming, as those
      of seals, sea turtles, whales, etc.

   2. (Naut.) The hand. [Slang]

Source : WordNet®

flipper
     n 1: a shoe for swimming; the paddle-like front is an aid in
          swimming (especially underwater) [syn: {flippers}, {fin},
           {fins}]
     2: the flat broad limb of aquatic animals specialized for
        swimming