The Collaborative International Dictionary of English v.0.48:

Weakness \Weak"ness\, n.
   1. The quality or state of being weak; want of strength or
      firmness; lack of vigor; want of resolution or of moral
      strength; feebleness.
      [1913 Webster]

   2. That which is a mark of lack of strength or resolution; a
      fault; a defect.
      [1913 Webster]

            Many take pleasure in spreading abroad the weakness
            of an exalted character.              --Spectator.
      [1913 Webster]

   Syn: Feebleness; debility; languor; imbecility; infirmness;
        infirmity; decrepitude; frailty; faintness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

weakness
    n 1: a flaw or weak point; "he was quick to point out his wife's
         failings" [syn: failing, weakness]
    2: powerlessness revealed by an inability to act; "in spite of
       their weakness the group remains active" [syn:
       helplessness, weakness, impuissance]
    3: the property of lacking physical or mental strength;
       liability to failure under pressure or stress or strain; "his
       weakness increased as he became older"; "the weakness of the
       span was overlooked until it collapsed" [ant: strength]
    4: the condition of being financially weak; "the weakness of the
       dollar against the yen" [ant: strength]
    5: a penchant for something even though it might not be good for
       you; "he has a weakness for chocolate"

Moby Thesaurus II by Grady Ward, 1.0:

400 Moby Thesaurus words for "weakness":
   Amor, Christian love, Eros, Platonic love, a thing for, abulia,
   accessibility, admiration, adoration, affection, affinity, agape,
   airiness, airy texture, amenableness, anemia, aptitude, aptness,
   ardency, ardor, area, ashiness, assailability, attachment,
   attenuation, bad habit, bag, bent, besetting sin, bias, bleariness,
   blemish, bloodlessness, blur, blurriness, bodily love, brain fag,
   brotherly love, bug, cadaverousness, carelessness, caritas, cast,
   catch, chance, charity, chicken-liveredness, chickenheartedness,
   chink, chloranemia, conatus, conduciveness, conjugal love,
   contingency, cowardice, cowardliness, crack, crush, cullibility,
   cup of tea, darkness, deadness, deathly hue, deathly pallor,
   debilitation, debility, deceivability, decrescendo, defect,
   defection, deficiency, defocus, delicacy, delight, desire,
   devotion, diaphanousness, diathesis, dilutedness, dilution,
   dimness, disposition, drawback, dullness, dupability, eagerness,
   easiness, easygoingness, enervation, enfeeblement, ethereality,
   eventuality, exiguity, exility, exsanguination, eyestrain,
   fadedness, failing, failure, faintheart, faintheartedness,
   faintness, fairness, faithful love, fancy, fatigue, fault, faute,
   fear, feeblemindedness, feebleness, feeling for, fervor, field,
   filminess, fineness, flabbiness, flame, flatness, flavorlessness,
   flaw, flimsiness, fogginess, foible, fondness, forcelessness,
   forte, frailty, free love, free-lovism, fuzziness, gauziness,
   gentleness, ghastliness, goneness, gossameriness, gracility,
   greenness, gullibility, gust, gusto, haggardness, half-visibility,
   haziness, heart, heart strain, heel of Achilles, henheartedness,
   hero worship, hoaxability, hole, human equation, human frailty,
   human weakness, humanity, humanness, hypochromia,
   hypochromic anemia, idolatry, idolism, idolization, imperfection,
   impotence, impotency, imprecision, impressionability, impuissance,
   inadequacy, inanity, inclination, indefiniteness, indifference,
   indistinctness, indistinguishability, ineffectiveness,
   ineffectuality, inefficaciousness, inefficacy, infatuation,
   infirmity, influenceability, ingenuousness, insipidity,
   insipidness, insubstantiality, jadedness, jejuneness, jejunity,
   kink, laciness, lack of force, lack of influence,
   lack of magnetism, lack of personality, lack of power, languor,
   lasciviousness, lassitude, laxity, laxness, leaning, leniency,
   liability, liableness, libido, lifelessness, lightness, like,
   likelihood, likeliness, likes, liking, lily-liveredness, line,
   little problem, lividness, long suit, looseness, loosening, love,
   lovemaking, low profile, lowness, main interest, malleability,
   manner, married love, mental fatigue, mental strain, metier,
   mildness, milksopism, milksoppiness, milksoppishness, mistiness,
   moral flaw, mortality, movability, muddiness, naiveness, naivete,
   naivety, negligence, no say, obligation, obscurity,
   open-mindedness, openness, overindulgence, overpermissiveness,
   overstrain, overtiredness, paleness, pallidity, pallidness, pallor,
   paperiness, passion, pastiness, penchant, penetrability,
   permissiveness, persuadability, perviousness, pet subject,
   physical love, pigeonheartedness, pliability, popular regard,
   popularity, possibility, power vacuum, powerlessness, predilection,
   predisposition, pregnability, prejudice, prison pallor,
   probability, problem, proclivity, proneness, propensity, pursuit,
   rarity, readiness, receptiveness, regard, relaxation, relaxedness,
   relish, remissness, responsiveness, rift, sallowness, saplessness,
   savorlessness, seduceability, semivisibility, sensitivity to,
   sentiment, sex, sexual love, shadowiness, shine, shortcoming,
   sickliness, sickly hue, simpleness, simplicity, slackness,
   sleepiness, slenderness, slightness, slimness, sloppiness, snag,
   soft focus, soft spot, softheartedness, softness,
   something missing, specialism, speciality, specialization,
   specialty, spinelessness, spiritual love, staleness,
   stance fatigue, strain, strong point, style, subaudibility,
   subduedness, subtility, suggestibility, susceptibility,
   swayableness, taint, taste, tastelessness, technicality, tendency,
   tender feeling, tender passion, tenderheartedness, tenderness,
   tenuity, thinness, timidity, timidness, timorousness, tiredness,
   tropism, truelove, turn, twist, type, unauthoritativeness,
   uncertainty, unclearness, uninfluentiality, unmanfulness,
   unmanliness, unpersuasiveness, unplainness, unrestraint,
   unsavoriness, unsophistication, uxoriousness, vague appearance,
   vagueness, vapidity, vice, vincibility, vocation, vulnerability,
   vulnerable place, vulnerable point, wanness, warm heart,
   warmheartedness, warp, wateriness, way, weak link, weak point,
   weak side, weak will, weak-mindedness, weakheartedness,
   wearifulness, weariness, willingness, wishy-washiness, wispiness,
   worship, yearning, yellowness