The Collaborative International Dictionary of English v.0.48:

Humorless \Hu"mor*less\, a.
   Destitute of humor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

humorless
    adj 1: lacking humor; "it was a humorless wink; a wink of
           warning"- Truman Capote [syn: humorless, humourless,
           unhumorous] [ant: humorous, humourous]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "humorless":
   cheerless, discontented, grim, in bad humor, infestive, joyless,
   mirthless, miserable, out of humor, out of sorts, pleasureless,
   sorry, sorryish, uncheerful, uncheery, unhappy, unjoyful,
   unmirthful, unsmiling, wretched