The Collaborative International Dictionary of English v.0.48:

Spineless \Spine"less\, a.
   1. Having no spine, or vertebral column.
      [1913 Webster]

   2. Having no thorns or barbs.
      [PJC]

   3. lacking in strength of character; cowardly, in a physical
      or moral sense.
      [PJC]

WordNet (r) 3.0 (2006):

spineless
    adj 1: weak in willpower, courage or vitality [syn: namby-
           pamby, gutless, spineless, wishy-washy]
    2: lacking a backbone or spinal column; "worms are an example of
       invertebrate animals" [syn: invertebrate, spineless]
       [ant: vertebrate]
    3: lacking spiny processes; "spineless fins" [ant: spinous,
       spiny]
    4: lacking thorns [syn: thornless, spineless]

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "spineless":
   abulic, adrift, afloat, afraid, alternating, amorphous, anemic,
   asthenic, bloodless, capricious, changeable, changeful, chicken,
   cowardly, craven, dastardly, debilitated, desultory, deviable,
   dizzy, drooping, droopy, dull, eccentric, effete, emasculate,
   erratic, etiolated, faint, fainthearted, faintish, fast and loose,
   fearful, feeble, feebleminded, fickle, fitful, flabby, flaccid,
   flickering, flighty, flitting, floppy, fluctuating, forceless,
   frail, freakish, giddy, gone, gutless, imbecile, impetuous,
   impotent, impulsive, inadequate, inconsistent, inconstant,
   indecisive, ineffective, ineffectual, infirm, invertebrate,
   irregular, irresolute, irresponsible, languid, languorous,
   lily-livered, limber, limp, listless, lustless, marrowless, mazy,
   mercurial, moody, nerveless, pithless, pliable, pooped, powerless,
   pusillanimous, rambling, restless, roving, rubbery, sapless,
   scatterbrained, shapeless, shifting, shifty, shuffling, sinewless,
   slack, soft, spasmodic, spiritless, squeamish, strengthless, timid,
   timorous, unaccountable, uncertain, uncontrolled, undependable,
   undisciplined, unfixed, unhardened, unnerved, unpredictable,
   unreliable, unrestrained, unsettled, unstable, unstable as water,
   unstaid, unsteadfast, unsteady, unstrung, vacillating, vagrant,
   variable, vicissitudinary, vicissitudinous, volatile, wandering,
   wanton, wavering, wavery, wavy, wayward, weak, weak-kneed,
   weak-minded, weak-willed, weakly, whimsical, white-livered,
   wishy-washy, yellow