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]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "ennui":
   acedia, angst, anguish, anxiety, apathy, blues, boredness, boredom,
   cheerlessness, dejection, depression, discomfort, discomposure,
   discontent, dislike, dispiritedness, displeasure, disquiet,
   dissatisfaction, doldrums, dread, drowsiness, dullness, dumps,
   emptiness, enervation, existential woe, fatigue, fed-upness,
   flatness, grimness, heaviness, hebetude, inanimation, indifference,
   inquietude, jadedness, joylessness, lack of pleasure,
   lackadaisicalness, languidness, languishment, languor,
   languorousness, lassitude, lenitude, lentor, lethargy,
   life-weariness, lifelessness, listlessness, malaise, melancholy,
   nausea, nongratification, nonsatisfaction, oscitancy, painfulness,
   pall, passivity, phlegm, sadness, satedness, satiation, satiety,
   savorlessness, sleepiness, sloth, slothfulness, slowness,
   sluggishness, somnolence, spiritlessness, spleen, staleness,
   stupefaction, stupor, supineness, surfeit, taedium vitae,
   tastelessness, tediousness, tedium, tire, tiredness, torpidity,
   torpidness, torpitude, torpor, uncomfortableness, unease,
   uneasiness, unhappiness, unpleasure, unsatisfaction,
   vexation of spirit, wearifulness, weariness, weary,
   world-weariness, yawn