The Collaborative International Dictionary of English v.0.48:

Ennui \En`nui"\, n. [F., fr. L. in odio in hatred. See Annoy.]
   A feeling of weariness and disgust; dullness and languor of
   spirits, arising from satiety or want of interest; tedium.
   --T. Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ennui
    n 1: the feeling of being bored by something tedious [syn:
         boredom, ennui, tedium]