mid-13c., departen, "part from each other, part company;" late 13c., "separate into parts," original senses now archaic or obsolete, from Old French departir (10c.) "to divide, distribute; separate (oneself), depart; die," from Late Latin departire "to divide" (transitive), from de- "from" (see de-) + partire "to part, divide," from pars (genitive partis) "a part, piece, a share, a division" (from PIE root *pere- (2) "to grant, allot").
From c. 1300 as "go or move away, withdraw;" late 14c. as "leave, quit." As a euphemism for "to die" (depart this life "leave the world;" compare Old French departir de cest siecle) it is attested from c. 1500, as is the departed for "the dead," singly or collectively. The original transitive lingered in some modern English usages; until 1662 the wedding service was till death us depart. Related: Departed; departing.