mid-14c., "deadly," also "doomed to die," from Old French mortel "destined to die; deserving of death," from Latin mortalis "subject to death, mortal, of a mortal, human," from mors (genitive mortis) "death," from PIE base *mer- "to die," with derivatives referring to death and human beings" (cognates: Sanskrit mrtih "death," martah "mortal man;" Avestan miryeite "dies," Old Persian martiya- "man;" Armenian meranim "die;" Latin mori "to die;" Lithuanian mirtis "mortal man;" Greek brotos "mortal" (hence ambrotos "immortal"); Old Church Slavonic mrutvu "dead;" Old Irish marb, Welsh marw "died;" Old English morþ "murder"). The most widespread Indo-European root for "to die," forming the common word for it except in Greek and Germanic. Watkins says it is "possibly" the same as PIE *mer- "rub, pound, wear away" (see morbid).

"mortal thing or substance," 1520s, from mortal, adj.. Latin mortalis also was used as a noun, "a man, mortal, human being."