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Origin of ‘Essential’
Essential has been part of English since the 14th century, and may be traced to the Latin word for essence (essentia). The earliest sense of the word in English was, unsurprisingly, strongly connected to its Latin origin; essential first meant “of, relating to, or constituting essence”. The common modern use (“of the utmost importance”) began to be used in the 16th century.
Words for “Not Essential”
Nonessential, unessential, and inessential appear to have entered use in the first half of the 17th century; all of these words in early use carried the meaning npt essential