Ennui means boredom in French, in the sense of absence of stimulation and sensation of discontent, especially in intellectual terms. It is different from the kind of boredom a child could feel and is closer to a sense of pointlessness, emptiness and existential angst brought by this emotion.
The term ennui was especially popularized in the English speaking world due to the work of French poet Charles Baudelaire
and his collection of poems Les fleurs du mal
, which thoroughly investigate the concept.
The word ennui is a collective term so it does not have a plural form.
The correct pronunciation of ennui is ohn-wee
The first syllable contains a nasal "n" that can be difficult to pronounce for a non-native speaker but is not absolutely essential to achieve a good pronunciation. It is more important to remember to pronounce the vowels in a rather closed manner, as you can hear in the audio pronunciation.