This word is almost as hard to define as it is to pronounce. It's a bit like being tired, but not quite. It's also a bit like being apathetic, but not quite that either. It's somewhere between the two, and at the same time somehow more artistic. "Ennui" is best defined as a feeling of something lacking, being energy, spirit or interest. Another way to put it could be an extreme feeling of discontentment. Whatever it is, it's not great!
Look at the word and try to get your head around it. How do you say it out loud? "EN-OO-EYE", right? Nope. Try again. Did you go with "EN-OO-EE"? Good effort, but you're still off target. It's "On-wee". "Ennui" is actually a French word by origin, and the French really love to go deep on those vowels.