The Collaborative International Dictionary of English v.0.48:

Fail \Fail\, n. [OF. faille, from failir. See Fail, v. i.]
   1. Miscarriage; failure; deficiency; fault; -- mostly
      superseded by failure or failing, except in the phrase
      without fail. "His highness' fail of issue." --Shak.
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   2. Death; decease. [Obs.] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Failing \Fail"ing\, n.
   1. A failing short; a becoming deficient; failure;
      deficiency; imperfection; weakness; lapse; fault;
      infirmity; as, a mental failing.
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            And ever in her mind she cast about
            For that unnoticed failing in herself. --Tennyson.
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   2. The act of becoming insolvent of bankrupt.

   Syn: See Fault.
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The Collaborative International Dictionary of English v.0.48:

Fail \Fail\ (f[=a]l) v. i. [imp. & p. p. Failed (f[=a]ld); p.
   pr. & vb. n. Failing.] [F. failir, fr. L. fallere, falsum,
   to deceive, akin to E. fall. See Fail, and cf. Fallacy,
   False, Fault.]
   1. To be wanting; to fall short; to be or become deficient in
      any measure or degree up to total absence; to cease to be
      furnished in the usual or expected manner, or to be
      altogether cut off from supply; to be lacking; as, streams
      fail; crops fail.
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            As the waters fail from the sea.      --Job xiv. 11.
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            Till Lionel's issue fails, his should not reign.
                                                  --Shak.
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   2. To be affected with want; to come short; to lack; to be
      deficient or unprovided; -- used with of.
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            If ever they fail of beauty, this failure is not be
            attributed to their size.             --Berke.
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   3. To fall away; to become diminished; to decline; to decay;
      to sink.
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            When earnestly they seek
            Such proof, conclude they then begin to fail.
                                                  --Milton.
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   4. To deteriorate in respect to vigor, activity, resources,
      etc.; to become weaker; as, a sick man fails.
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   5. To perish; to die; -- used of a person. [Obs.]
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            Had the king in his last sickness failed. --Shak.
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   6. To be found wanting with respect to an action or a duty to
      be performed, a result to be secured, etc.; to miss; not
      to fulfill expectation.
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            Take heed now that ye fail not to do this. --Ezra
                                                  iv. 22.
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            Either my eyesight fails, or thou look'st pale.
                                                  --Shak.
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   7. To come short of a result or object aimed at or desired;
      to be baffled or frusrated.
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            Our envious foe hath failed.          --Milton.
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   8. To err in judgment; to be mistaken.
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            Which ofttimes may succeed, so as perhaps
            Shall grieve him, if I fail not.      --Milton.
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   9. To become unable to meet one's engagements; especially, to
      be unable to pay one's debts or discharge one's business
      obligation; to become bankrupt or insolvent; as, many
      credit unions failed in the late 1980's.
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WordNet (r) 3.0 (2006):

failing
    adj 1: below acceptable in performance; "received failing
           grades"
    n 1: a flaw or weak point; "he was quick to point out his wife's
         failings" [syn: failing, weakness]
    2: failure to reach a minimum required performance; "his failing
       the course led to his disqualification"; "he got two flunks
       on his report" [syn: failing, flunk] [ant: pass,
       passing, qualifying]

Moby Thesaurus II by Grady Ward, 1.0:

202 Moby Thesaurus words for "failing":
   abortive, arrested, bad habit, besetting sin, blemish, blind spot,
   bootless, bug, cachectic, callow, catch, comedown, coming apart,
   crack, cracking, crumbling, debasement, debilitated, decadence,
   decadency, decadent, declension, declination, decline, declining,
   defect, defection, defective, deficiency, deficient, deformation,
   degeneracy, degenerate, degenerateness, degeneration, degradation,
   demotion, depravation, depravedness, depreciation, derogation,
   descent, deteriorating, deterioration, devolution, disintegrating,
   downtrend, downturn, downward mobility, downward trend, drained,
   draining, drawback, drooping, drop, dwindling, dying, ebb, ebbing,
   effete, effeteness, embryonic, enervated, exhausted, fading,
   failed, failure, failure of nerve, fall, falling, falling-off,
   fault, faute, feeble, flagging, flaw, foible, found wanting,
   fragmenting, frail, frailty, fruitless, futile, going to pieces,
   healthless, hole, hypoplastic, immature, imperfect, imperfection,
   in arrear, in arrears, in default, in default of, in poor health,
   in short supply, inadequacy, inadequate, incompetent, incomplete,
   ineffective, ineffectual, inefficacious, infant, infirm, infirmity,
   insufficient, invalid, involution, kink, lacking, lame,
   languishing, lapse, little problem, loss of tone, manque,
   marcescent, miscarried, miscarrying, missing, moral flaw, moribund,
   needing, not enough, of no effect, pale, part, partial, patchy,
   peaked, peaky, pining, problem, reduced, reduced in health,
   regression, regressive, retrocession, retrogradation, retrograde,
   retrogression, retrogressive, rift, run-down, sans, scant, scanty,
   scarce, scrappy, short, shortcoming, shriveling, shy, sickly,
   sinking, sketchy, sliding, slippage, slipping, slump, slumping,
   snag, something missing, stickit, stillborn, subsiding,
   successless, tabetic, taint, too little, underdeveloped,
   undeveloped, unequal to, unfortunate, unhealthy, unqualified,
   unsatisfactory, unsatisfying, unsound, unsuccessful, unsufficing,
   useless, valetudinarian, valetudinary, vice, vulnerable place,
   wane, waning, wanting, wasting, weak link, weak point, weak side,
   weakened, weakly, weakness, wilting, with low resistance,
   withering, without, worsening