The Collaborative International Dictionary of English v.0.48:

Spiritless \Spir"it*less\, a.
   1. Destitute of spirit; wanting animation; wanting
      cheerfulness; dejected; depressed.
      [1913 Webster]

   2. Destitute of vigor; wanting life, courage, or fire.
      [1913 Webster]

            A men so faint, so spiritless,
            So dull, so dead in lock, so woebegone. --Shak.
      [1913 Webster]

   3. Having no breath; extinct; dead. "The spiritless body."
      --Greenhill.
      [1913 Webster] -- Spir"it*less*ly, adv. --
      Spir"it*less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spiritless
    adj 1: lacking ardor or vigor or energy; "a spiritless reply to
           criticism" [ant: spirited]
    2: evidencing little spirit or courage; overly submissive or
       compliant; "compliant and anxious to suit his opinions of
       those of others"; "a fine fiery blast against meek
       conformity"- Orville Prescott; "she looked meek but had the
       heart of a lion"; "was submissive and subservient" [syn:
       meek, spiritless]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "spiritless":
   Laodicean, Olympian, affectless, aloof, anesthetized, apathetic,
   arctic, arid, asleep, autistic, barren, benumbed, blah, blank,
   blase, bloodless, blue, blunt, bowed-down, cast down, catatonic,
   characterless, chill, chilly, cold, cold as charity, cold-blooded,
   coldhearted, colorless, comatose, cool, dashed, dead, deceased,
   defunct, dejected, departed, depressed, desensitized, despairing,
   despondent, desponding, detached, disconsolate, discouraged,
   disheartened, disinterested, dismal, dispassionate, dispirited,
   down, downcast, downhearted, draggy, drearisome, dreary, drooping,
   droopy, drugged, dry, dryasdust, dull, dusty, effete, elephantine,
   emotionally dead, emotionless, empty, etiolated, exanimate,
   extinct, fade, feeling low, flat, frigid, frosted, frosty, frozen,
   gritless, gutless, heartless, heavy, hebetudinous, ho-hum, hollow,
   hopeless, hypochondriac, hypochondriacal, icy, immovable,
   impassible, impassive, in a stupor, in low spirits, in the depths,
   in the doldrums, in the dumps, inane, indifferent, inexcitable,
   insipid, insouciant, insusceptible, jejune, languid, languishing,
   leaden, lethargic, lifeless, listless, low, low-spirited,
   nonchalant, nonemotional, numb, numbed, objective, obtuse,
   out of touch, pale, pallid, passionless, passive, pedestrian,
   pessimistic, phlegmatic, pining, plodding, pluckless, pointless,
   poky, ponderous, resigned, self-absorbed, slack, slow, sluggish,
   solemn, soporific, soulless, spunkless, sterile, stiff, stodgy,
   stoic, stuffy, stupefied, subdued, submissive, suicidal,
   superficial, supine, tasteless, tedious, torpid, unaffectionate,
   unanimated, uncaring, unconcerned, uncourageous, undaring,
   unemotional, unfeeling, ungallant, unheroic, unimpassioned,
   unimpressionable, uninterested, unintrepid, unlively, unloving,
   unpassionate, unresponding, unresponsive, unsoldierlike,
   unsoldierly, unsusceptible, unsympathetic, untouchable, unvaliant,
   unvalorous, vapid, weary of life, withdrawn, woebegone, wooden,
   world-weary