The Collaborative International Dictionary of English v.0.48:

Tantalus \Tan"ta*lus\, n. [L., from Gr. Ta`ntalos.] (Gr. Myth.)
   1. A Phrygian king who was punished in the lower world by
      being placed in the midst of a lake whose waters reached
      to his chin but receded whenever he attempted to allay his
      thirst, while over his head hung branches laden with
      choice fruit which likewise receded whenever he stretched
      out his hand to grasp them.
      [1913 Webster]

   2. (Zool.) A genus of wading birds comprising the wood
      ibises.
      [1913 Webster]

   Tantalus's cup (Physics), a philosophical toy, consisting
      of a cup, within which is the figure of a man, and within
      the figure a siphon, the longer arm of which passes down
      through the bottom of the cup, and allows the escape of
      any liquid that may be poured in, when it reaches as high
      as the bend of the siphon, which is just below the level
      of the mouth of the figure in the cup.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Tantalus
    n 1: (Greek mythology) a wicked king and son of Zeus; condemned
         in Hades to stand in water that receded when he tried to
         drink and beneath fruit that receded when he reached for it