Source : Webster's Revised Unabridged Dictionary (1913)

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.

         The most dark, tenebrous night.          --J. Hall
                                                  (1565).

         The towering and tenebrous boughts of the cypress.
                                                  --Longfellow.

Source : WordNet®

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