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mid-13c., "part from each other," from Old French departir (10c.) "to divide, distribute; separate (oneself), depart; die," from Late Latin departire "divide" (transitive), from de- "from" (see de-) + partire "to part, divide," from pars (genitive partis) "a part" (see part (n.)).
As a euphemism for "to die" (to depart this life; cf. Old French departir de cest siecle) it is attested from c.1500, as is the departed for "the dead," singly or collectively. Transitive lingers in some English usages; the wedding service was till death us depart until 1662. Related: Departed; departing.