Source : Webster's Revised Unabridged Dictionary (1913)

Lack \Lack\, v. t. [imp. & p. p. {Lacked}; p. pr. & vb. n.
   {Lacking}.]
   1. To blame; to find fault with. [Obs.]

            Love them and lakke them not.         --Piers
                                                  Plowman.

   2. To be without or destitute of; to want; to need.

            If any of you lack wisdom, let him ask of God.
                                                  --James i. 5.

Source : WordNet®

lacking
     adj 1: not existing; "innovation has been sadly lacking";
            "character development is missing from the book" [syn:
             {lacking(p)}, {missing}, {nonexistent}, {wanting(a)}]
     2: 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)}]