The word ennui comes from French and originally meant "annoyance" or "vexation".   It traces back to the Old French word enui, from the verb enuiier (or enuier), which in turn derives from the Late Latin inodiō ("hated"), the same root that gave English the word "annoy". English borrowed the term in the 17th century, and it evolved to describe a more specific kind of boredom: a feeling of listlessness, dissatisfaction, or apathy resulting from a lack of excitement or occupation. 

Evolution of the word: Late Latin: The root is the Late Latin phrase inodiō, meaning "hated".  Old French: This evolved into the Old French word enui and the verb enuiier (to annoy).  English: The word was borrowed into English in the 1660s as a direct loan from French, initially as ennui.  Meaning shift: While its Latin and French roots are tied to "annoyance," the English usage shifted to a more profound sense of boredom, weariness, or apathy, particularly when life becomes too easy or monotonous.