The Collaborative International Dictionary of English v.0.48:

Lifeless \Life"less\, a.
   Destitute of life, or deprived of life; not containing, or
   inhabited by, living beings or vegetation; dead, or
   apparently dead; spiritless; powerless; dull; as, a lifeless
   carcass; lifeless matter; a lifeless desert; a lifeless wine;
   a lifeless story. -- Life"less*ly, adv. --
   Life"less*ness, n.

   Syn: Dead; soulless; inanimate; torpid; inert; inactive;
        dull; heavy; unanimated; spiritless; frigid; pointless;
        vapid; flat; tasteless.

   Usage: Lifeless, Dull, Inanimate, Dead. In a moral
          sense, lifeless denotes a lack of vital energy;
          inanimate, a lack of expression as to any feeling that
          may be possessed; dull implies a torpor of soul which
          checks all mental activity; dead supposes a
          destitution of feeling. A person is said to be
          lifeless who has lost the spirits which he once had;
          he is said to be inanimate when he is naturally
          wanting in spirits; one is dull from an original
          deficiency of mental power; he who is dead to moral
          sentiment is wholly bereft of the highest attribute of
          his nature.
          [1913 Webster]

WordNet (r) 3.0 (2006):

lifeless
    adj 1: deprived of life; no longer living; "a lifeless body"
           [syn: lifeless, exanimate]
    2: destitute or having been emptied of life or living beings;
       "after the dance the littered and lifeless ballroom echoed
       hollowly"
    3: lacking animation or excitement or activity; "the party being
       dead we left early"; "it was a lifeless party until she
       arrived"
    4: not having the capacity to support life; "a lifeless planet"