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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lacking
    adj 1: inadequate in amount or degree; "a deficient education";
           "deficient in common sense"; "lacking in stamina";
           "tested and found wanting" [syn: deficient,
           lacking(p), wanting(p)]
    2: nonexistent; "the thumb is absent"; "her appetite was
       lacking" [syn: lacking, absent, missing, wanting]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "lacking":
   absconded, absent, adulterated, arrested, away, bankrupt in,
   bare of, bereaved, bereaved of, bereft, bereft of, blemished,
   callow, cut off, damaged, defective, deficient, deleted, denuded,
   denuded of, departed, deprived of, destitute of, devoid, devoid of,
   disappeared, divested, embryonic, empty of, erroneous, existless,
   failing, fallible, faulty, for want of, forlorn of, found wanting,
   gone, half-assed, hypoplastic, immature, impaired, imperfect,
   imprecise, impure, in arrear, in arrears, in default,
   in default of, in short supply, in want of, inaccurate, inadequate,
   incompetent, incomplete, inexact, infant, inferior, insufficient,
   makeshift, mediocre, minus, missing, mixed, needing, negative,
   no longer present, nonattendant, nonexistent, not enough,
   not found, not perfect, not present, null, off, omitted, out of,
   out of pocket, out of sight, part, parted from, partial, patchy,
   robbed of, sans, scant, scant of, scanty, scrappy, shorn of, short,
   short of, shy, shy of, sketchy, stripped of, subtracted,
   taken away, too little, unblessed with, underdeveloped,
   undeveloped, unequal to, uneven, unexisting, unfinished,
   unperfected, unpossessed of, unqualified, unreached,
   unsatisfactory, unsatisfying, unsound, unsufficing, unthorough,
   vacuous, vanished, void, void of, wanting, without being