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]