The Collaborative International Dictionary of English v.0.48:

Jew's-harp \Jew's-harp`\, n. [Jew + harp; or possibly a corrupt.
   of jaw's harp; cf. G. maultrommel, lit., mouthdrum.]
   1. An instrument of music, which, when placed between the
      teeth, gives, by means of a bent metal tongue struck by
      the finger, a sound which is modulated by the breath; --
      called also Jew's-trump.
      [1913 Webster]

   2. (Naut.) The shackle for joining a chain cable to an
      anchor.
      [1913 Webster] Jew's-stone

The Devil's Dictionary (1881-1906):

JEWS-HARP, n.  An unmusical instrument, played by holding it fast with
the teeth and trying to brush it away with the finger.