Source : Webster's Revised Unabridged Dictionary (1913)

Theorbo \The*or"bo\, n. [F. th['e]orbe, t['e]orbe, formerly
   tuorbe, tiorbe, It. tiorba.] (Mus.)
   An instrument made like large lute, but having two necks,
   with two sets of pegs, the lower set holding the strings
   governed by frets, while to the upper set were attached the
   long bass strings used as open notes.

   Note: A larger form of theorbo was also called the
         {archlute}, and was used chiefly, if not only, as an
         accompaniment to the voice. Both have long fallen into
         disuse.