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late 15c., past participle adjective from decease (v.). As a verbal noun meaning "dead person, those who are dead," from early 17c.
"death," early 14c., from Old French deces (12c., Modern French décès) "decease, death," from Latin decessus "death" (euphemism for mors), also "a retirement, a departure," from decess-, past participle stem of decedere "die, depart, withdraw," literally "to go down," from de- "away" (see de-) + cedere "go" (see cede). Still used with a tinge of euphemism.
"to die," early 15c., from decease (n.). Related: Deceased; deceasing