The Collaborative International Dictionary of English v.0.48:

adynamic \ad`y*nam"ic\ ([a^]d`[i^]*n[a^]m"[i^]k), a. [Cf. F.
   adynamique. See Adynamy.]
   1. (Med.) Pertaining to, or characterized by adynamia; weak.
      [1913 Webster + AS]

   2. (Physics) Characterized by the absence of power or force.
      [1913 Webster]

   Adynamic fevers, malignant or putrid fevers attended with
      great muscular debility.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adynamic
    adj 1: characterized by an absence of force or forcefulness
           [syn: undynamic, adynamic] [ant: dynamic,
           dynamical]
    2: lacking strength or vigor [syn: adynamic, asthenic,
       debilitated, enervated]