ennui, boredom; lassitude (fatigue) [See Fatigue]

1.
"I sometimes feel a great ennui, profound emptiness, doubts which sneer in my face in the midst of the most spontaneous satisfactions."

--- Flaubert

2.
"This Ennui, for which we Saxons had no name, this word of France has got a terrific significance. It shortens life, and bereaves the day of its light."

--- Emerson