The Collaborative International Dictionary of English v.0.48:

Atony \At"o*ny\, n. [Gr. ? slackness; 'a priv. + ? tone,
   strength, ? to stretch: cf. F. atonie.] (Med.)
   Want of tone; weakness of the system, or of any organ,
   especially of such as are contractile.
   [1913 Webster]

WordNet (r) 3.0 (2006):

atony
    n 1: lack of normal muscular tension or tonus [syn: atonicity,
         atony, atonia, amyotonia] [ant: tone, tonicity,
         tonus]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "atony":
   adynamia, anemia, blah feeling, bloodlessness, cachexia, cachexy,
   cowardice, debilitation, debility, dullness, etiolation, faintness,
   fatigue, feebleness, flabbiness, flaccidity, impotence,
   languishment, languor, lassitude, listlessness, prostration,
   sluggishness, softness, strengthlessness, weakliness, weakness,
   weariness