The Collaborative International Dictionary of English v.0.48:

Debility \De*bil"i*ty\, n. [L. debilitas, fr. debilis weak,
   prob. fr. de- + habilis able: cf. F. d['e]bilit['e]. See
   Able, a.]
   The state of being weak; weakness; feebleness; languor.
   [1913 Webster]

         The inconveniences of too strong a perspiration, which
         are debility, faintness, and sometimes sudden death.
                                                  --Arbuthnot.

   Syn: Debility, Infirmity, Imbecility.

   Usage: An infirmity belongs, for the most part, to particular
          members, and is often temporary, as of the eyes, etc.
          Debility is more general, and while it lasts impairs
          the ordinary functions of nature. Imbecility attaches
          to the whole frame, and renders it more or less
          powerless. Debility may be constitutional or may be
          the result or superinduced causes; Imbecility is
          always constitutional; infirmity is accidental, and
          results from sickness or a decay of the frame. These
          words, in their figurative uses, have the same
          distinctions; we speak of infirmity of will, debility
          of body, and an Imbecility which affects the whole
          man; but Imbecility is often used with specific
          reference to feebleness of mind.
          [1913 Webster]

WordNet (r) 3.0 (2006):

debility
    n 1: the state of being weak in health or body (especially from
         old age) [syn: infirmity, frailty, debility,
         feebleness, frailness, valetudinarianism]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "debility":
   advanced age, advanced years, adynamia, age of retirement,
   an incurable disease, anemia, anility, atony, blah feeling,
   bloodlessness, brain fag, cachexia, cachexy, caducity,
   chronic ill health, cowardice, debilitation, decline of life,
   declining years, decrepitude, delicacy, delicate health, disease,
   dotage, dullness, eld, elderliness, enervation, enfeeblement,
   etiolation, exhaustion, eyestrain, faintness, fatigue, feebleness,
   flabbiness, flaccidity, fragility, frailty, goneness,
   green old age, hale old age, healthlessness, heart strain,
   hoary age, hypochondria, hypochondriasis, ill health, impotence,
   infirm old age, infirmity, infirmity of age, invalidism,
   invalidity, jadedness, languishing, languishment, languor,
   lassitude, listlessness, longevity, malaise, mental fatigue,
   mental strain, morbidity, morbidness, old age, oldness, overstrain,
   overtiredness, peakedness, pensionable age, poor health,
   prostration, ricketiness, ripe old age, second childhood,
   senectitude, senility, senior citizenship, sickliness, sleepiness,
   sluggishness, softness, stance fatigue, strain, strengthlessness,
   superannuation, the downward slope, the golden years, tiredness,
   unhealthiness, unsoundness, unwholesomeness, vale of years,
   valetudinarianism, wasting, weakliness, weakness, wearifulness,
   weariness, white hairs