The Collaborative International Dictionary of English v.0.48:

Tasto \Tas"to\ (t[aum]s"t[-o]), n. [It.] (Mus.)
   A key or thing touched to produce a tone.
   [1913 Webster]

   Tasto solo, single touch; -- in old music, a direction
      denoting that the notes in the bass over or under which it
      is written should be performed alone, or with no other
      chords than unisons and octaves.
      [1913 Webster]