The Collaborative International Dictionary of English v.0.48:

Spinet \Spin"et\, n. [OF. espinete, F. ['e]pinette (cf. It.
   spinetta), fr. L. spina a thorn; -- so called because its
   quills resemble thorns. See Spine.] (Mus.)
   A keyed instrument of music resembling a harpsichord, but
   smaller, with one string of brass or steel wire to each note,
   sounded by means of leather or quill plectrums or jacks. It
   was formerly much used.
   [1913 Webster]

   Dumb spinet. (Mus.) See Manichordon.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Spinet \Spi"net\, n. [L. spinetum. See Spinny.]
   A spinny. [Obs.] --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spinet
    n 1: a small and compactly built upright piano
    2: early model harpsichord with only one string per note