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John Stuart Mill first used raison d’etre in English (1864)

The most important reason, the justification, or purpose for someone or something’s existence. What you live to do–or why you do (important) things. More colloquially, the “be-all and end-all”. 

From French: “reason for being”, which in turn comes from the Latin ratiō (reason) and esse (to be). First appears in English in a letter written in 1864 by John Stuart Mill: “Modes of speech which have a real raison d’être.” 

Still pronounced as in French: [rey-zohn de-truh] 

Compare mission, purpose, Ikaigi