A fancy word borrowed from the French, ennui (pronounced ahn-wee) is basically a synonym for boredom.
But as pointed out in an article from The Maven’s Word of the Day (see Google’s cached page), the word ennui has a more specialized meaning: “a feeling of utter weariness and discontent resulting from satiety or lack of interest”. So if you’re suffering from ennui, not only are you bored, but you’re worn out from either boredom or satiety, “the state of being satisfactorily full and unable to take on more”.
The phrase mihi in odio est (literally translated as "to me in a condition of dislike or hatred is"), meaning "I hate or dislike," gave rise to the Vulgar Latin verb *inodiare, "to make odious," the source of the Old French verb ennuyer or anoier, "to annoy, bore." This was borrowed into English by around 1275 as anoien, our annoy. From the Old French verb a noun meaning "worry, boredom" was derived, which became ennui in modern French. This noun, with the sense "boredom," was borrowed into English in the 18th century, perhaps filling a need in polite, cultivated society.
Polite, cultivated society is generally boring, so I can see why they’d need this word. Oh, and speaking of which, I’m sure I’ll have plenty of opportunities to use ennui when I start college. Looking forward to it! (Not really.)