The Collaborative International Dictionary of English v.0.48:

Tenebrous \Ten"e*brous\, a. [L. tenebrosus, fr. tenebrae
   darkness: cf. F. t['e]n['e]breux.]
   Dark; gloomy; dusky; tenebrious. -- Ten"e*brous*ness, n.
   [1913 Webster]

         The most dark, tenebrous night.          --J. Hall
                                                  (1565).
   [1913 Webster]

         The towering and tenebrous boughts of the cypress.
                                                  --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tenebrous
    adj 1: dark and gloomy; "a tenebrous cave" [syn: tenebrous,
           tenebrific, tenebrious]