The Collaborative International Dictionary of English v.0.48:

Lac \Lac\ (l[a^]k), Lakh \Lakh\ (l[aum]k), n. [Hind. lak,
   l[=a]kh, l[=a]ksh, Skr. laksha a mark, sign, lakh.]
   One hundred thousand; also, a vaguely great number; as, a lac
   of rupees. [Written also lack.] [East Indies]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lack \Lack\, v. i.
   1. To be wanting; often, impersonally, with of, meaning, to
      be less than, short, not quite, etc.
      [1913 Webster]

            What hour now?
            I think it lacks of twelve.           --Shak.
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            Peradventure there shall lack five of the fifty.
                                                  --Gen. xvii.
                                                  28.
      [1913 Webster]

   2. To be in want.
      [1913 Webster]

            The young lions do lack, and suffer hunger. --Ps.
                                                  xxxiv. 10.
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The Collaborative International Dictionary of English v.0.48:

Lack \Lack\ (l[a^]k), n. [OE. lak; cf. D. lak slander, laken to
   blame, OHG. lahan, AS. le['a]n.]
   1. Blame; cause of blame; fault; crime; offense. [Obs.]
      --Chaucer.
      [1913 Webster]

   2. Deficiency; want; need; destitution; failure; as, a lack
      of sufficient food.
      [1913 Webster]

            She swooneth now and now for lakke of blood.
                                                  --Chaucer.
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            Let his lack of years be no impediment. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Lack \Lack\, v. t. [imp. & p. p. Lacked (l[a^]kt); p. pr. &
   vb. n. Lacking.]
   1. To blame; to find fault with. [Obs.]
      [1913 Webster]

            Love them and lakke them not.         --Piers
                                                  Plowman.
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   2. To be without or destitute of; to want; to need.
      [1913 Webster]

            If any of you lack wisdom, let him ask of God.
                                                  --James i. 5.
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The Collaborative International Dictionary of English v.0.48:

Lack \Lack\, interj. [Cf. Alack.]
   Exclamation of regret or surprise. [Prov. Eng.] --Cowper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lack
    n 1: the state of needing something that is absent or
         unavailable; "there is a serious lack of insight into the
         problem"; "water is the critical deficiency in desert
         regions"; "for want of a nail the shoe was lost" [syn:
         lack, deficiency, want]
    v 1: be without; "This soup lacks salt"; "There is something
         missing in my jewelry box!" [syn: miss, lack] [ant:
         feature, have]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "lack":
   absence, adulteration, arrearage, awayness, bare cupboard,
   bare subsistence, be found wanting, be in want, be insufficient,
   be pinched, be poor, beggarliness, beggary, blank, break, collapse,
   come short, dearth, decline, defalcation, default, defect,
   defectibility, defectiveness, deficiency, deficit, deprivation,
   destitution, discontinuity, drought, empty purse, erroneousness,
   fail, fail of, fall away, fall short, fall shy, fallibility,
   famine, faultiness, gap, go on welfare, grinding poverty, gripe,
   hand-to-mouth existence, hiatus, homelessness, immaturity,
   impairment, imperfection, impoverishment, impurity, inaccuracy,
   inadequacy, inadequateness, incompleteness, indigence,
   inexactitude, inexactness, insufficiency, interval, kick the beam,
   lacuna, lag, lose ground, mediocrity, mendicancy, miss,
   missing link, moneylessness, necessitousness, necessity, need,
   neediness, neverness, nonexistence, nonoccurrence, nonpresence,
   not answer, not hack it, not make it, not make out, not measure up,
   not qualify, not stretch, not suffice, nowhereness, omission,
   outage, patchiness, paucity, pauperism, pauperization, penury,
   pinch, privation, require, run short, run short of, scantiness,
   scarcity, shortage, shortcoming, shortfall, sketchiness, slump,
   starvation, starve, stop short, subtraction, ullage, underage,
   undevelopment, unevenness, unperfectedness, unsoundness, want,
   wantage