The Collaborative International Dictionary of English v.0.48:

Lackluster \Lack"lus`ter\, Lacklustre \Lack"lus`tre\, n.
   A lack of luster.
   [1913 Webster] Lackluster

The Collaborative International Dictionary of English v.0.48:

Lackluster \Lack"lus`ter\, Lacklustre \Lack"lus`tre\ a.
   1. Wanting luster or brightness. "Lackluster eye." --Shak.
      [1913 Webster]

   2. Lacking spirit, liveliness, or enthusiasm; dull.
      [PJC]

   3. Mediocre; as, a lackluster performance.
      [PJC]

WordNet (r) 3.0 (2006):

lacklustre
    adj 1: lacking brilliance or vitality; "a dull lackluster life";
           "a lusterless performance" [syn: lackluster,
           lacklustre, lusterless, lustreless]
    2: lacking luster or shine; "staring with lackluster eyes";
       "lusterless hair" [syn: lackluster, lacklustre,
       lusterless, lustreless]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "lacklustre":
   blah, bland, boring, colorless, dense, dingy, dismal, drab, dreary,
   dull, flat, insipid, prosaic, slow, tedious, thick, tiresome,
   two-dimensional, unexciting, unimaginative, uninteresting, vapid,
   wearisome, wishy-washy