The Collaborative International Dictionary of English v.0.48:

Supine \Su*pine"\ (s[-u]*p[imac]n"), a. [L. supinus, akin to sub
   under, super above. Cf. Sub-, Super-.]
   1. Lying on the back, or with the face upward; -- opposed to
      prone.
      [1913 Webster]

   2. Leaning backward, or inclining with exposure to the sun;
      sloping; inclined.
      [1913 Webster]

            If the vine
            On rising ground be placed, or hills supine.
                                                  --Dryden.
      [1913 Webster]

   3. Negligent; heedless; indolent; listless.
      [1913 Webster]

            He became pusillanimous and supine, and openly
            exposed to any temptation.            --Woodward.
      [1913 Webster]

   Syn: Negligent; heedless; indolent; thoughtless; inattentive;
        listless; careless; drowsy.
        [1913 Webster] -- Su*pine"ly, adv. -- Su*pine"ness,
        n.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Supine \Su"pine\ (s[=u]"p[imac]n), n. [L. supinum (sc. verbum),
   from supinus bent or thrown backward, perhaps so called
   because, although furnished with substantive case endings, it
   rests or falls back, as it were, on the verb: cf. F. supin.]
   (Lat. Gram.)
   A verbal noun; or (according to C.F.Becker), a case of the
   infinitive mood ending in -um and -u, that in -um being
   sometimes called the former supine, and that in -u the latter
   supine.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supine
    adj 1: lying face upward [syn: supine, resupine]
    2: offering no resistance; "resistless hostages"; "No other
       colony showed such supine, selfish helplessness in allowing
       her own border citizens to be mercilessly harried"- Theodore
       Roosevelt [syn: resistless, supine, unresisting]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "supine":
   Laodicean, Olympian, abject, accepting, accumbent, acquiescent,
   agreeable, aloof, apathetic, assenting, benumbed, blah, blase,
   bored, comatose, complaisant, compliable, compliant, complying,
   consenting, couchant, couche, crawling, crouched, dead, debased,
   debilitated, decumbent, depressed, desensitized, detached,
   disinterested, dopey, dormant, draped, droopy, drugged, dull,
   enervated, exanimate, flat, groveling, heartless, heavy,
   hebetudinous, hopeless, idle, in a stupor, inactive, inanimate,
   indifferent, indolent, inert, insouciant, jaded, knee-high,
   knocked flat, lackadaisical, laid low, languid, languorous, lazy,
   leaden, lethargic, lifeless, listless, lolling, lounging, low,
   low-built, low-hung, low-level, low-leveled, low-lying, low-set,
   low-statured, lumpish, lying, lying down, moribund, motionless,
   neap, nonchalant, nondissenting, nonresistant, nonresisting,
   nonresistive, numb, numbed, obedient, passive, phlegmatic,
   pluckless, pooped, procumbent, prone, prostrate, reclining,
   recumbent, reposing, resigned, resupine, runty, sated, servile,
   short, slack, sleepy, slothful, slow, sluggish, somnolent,
   soporific, spiritless, sprawled, sprawling, spread, spunkless,
   squat, squatty, stagnant, stagnating, stoic, stooped, stultified,
   stumpy, stupefied, submissive, subservient, torpid, unassertive,
   uncaring, uncomplaining, unconcerned, unelevated, uninterested,
   unresistant, unresisting, vegetable, vegetative, wan, weary,
   withdrawn, world-weary