The Collaborative International Dictionary of English v.0.48:

Decumbent \De*cum"bent\, a. [L. decumbens, -entis, p. pr. of
   decumbere; de- + cumbere (only in comp.), cubare to lie
   down.]
   1. Lying down; prostrate; recumbent.
      [1913 Webster]

            The decumbent portraiture of a woman. --Ashmole.
      [1913 Webster]

   2. (Bot.) Reclining on the ground, as if too weak to stand,
      and tending to rise at the summit or apex; as, a decumbent
      stem. --Gray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decumbent
    adj 1: lying down; in a position of comfort or rest [syn:
           accumbent, decumbent, recumbent]