mid-14c., "condition of being mortal," from Old French mortalite "massacre, slaughter; fatal illness; poverty; destruction" (12c.), from Latin mortalitem (nominative mortalitas) "state of being mortal; subjection to death," from mortalis (see mortal, adj.). Meaning "widespread death" is from c.1400; meaning "number of deaths from some cause or in a given period" is from 1640s.