The Collaborative International Dictionary of English v.0.48:

Tuneless \Tune"less\ (t[=u]n"l[e^]s), a.
   1. Without tune; inharmonious; unmusical. " Thy tuneless
      serenade." --Cowley.
      [1913 Webster]

            How often have I led thy sportive choir,
            With tuneless pipe, beside the murmuring Loire!
                                                  --Goldsmith.
      [1913 Webster]

   2. Not employed in making music; as, tuneless harps.
      [1913 Webster]

   3. Not expressed in music or poetry; unsung. [R.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tuneless
    adj 1: not having a musical sound or pleasing tune [syn:
           tuneless, untuneful, unmelodious] [ant:
           melodious, tuneful]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "tuneless":
   absonant, atonal, cacophonous, cracked, diaphonic, disconsonant,
   discordant, disharmonic, disharmonious, dissonant, flat, grating,
   harsh, immelodious, inharmonic, inharmonious, musicless,
   nonmelodious, off, off-key, off-tone, out of pitch, out of tone,
   out of tune, raucous, sharp, shrill, sour, strident, unharmonious,
   unmelodious, unmusical, untunable, untuned, untuneful