Deceased

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Middle English: from Old French deces, from Latin decessus ‘death’, past participle (used as a noun) of decedere ‘to die’.

wiktionary

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From decease +‎ -ed, from Middle English deceas via Old French[Term?], from Latin dēcessus(“departure”), equivalent to dēced-, variation of dēcēdō, dēcēdere(“to go away”).

etymonline

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deceased (adj.)

mid-15c., "dead, departed from life," past-participle adjective from decease (v.). As a verbal noun meaning "dead person, those who are dead," from early 17c.