means What is left over after something is used, removed, destroyed, or has died — including a dead body or the ruins of something past.

from From the Latin 'remanere,' to stay behind, built from 're-' (back) plus 'manere' (to stay) — a cousin of 'mansion,' which is literally a 'staying place.' It reached English through Old French 'remaindre' in the medieval period, and the noun sense of 'leftovers' (and grimly, of corpses) grew naturally from the verb: what remains is simply what stayed when everything else departed.