The Collaborative International Dictionary of English v.0.48:

Wrest \Wrest\, n.
   1. The act of wresting; a wrench; a violent twist; hence,
      distortion; perversion. --Hooker.
      [1913 Webster]

   2. Active or moving power. [Obs.] --Spenser.
      [1913 Webster]

   3. A key to tune a stringed instrument of music.
      [1913 Webster]

            The minstrel . . . wore round his neck a silver
            chain, by which hung the wrest, or key, with which
            he tuned his harp.                    --Sir W.
                                                  Scott.
      [1913 Webster]

   4. A partition in a water wheel, by which the form of the
      buckets is determined.
      [1913 Webster]

   Wrest pin (Piano Manuf.), one of the pins around which the
      ends of the wires are wound in a piano. --Knight.

   Wrest plank (Piano Manuf.), the part in which the wrest
      pins are inserted.
      [1913 Webster]