mortal (n.) Look up mortal at Dictionary.com
"mortal thing or substance," 1520s, from mortal (adj.). Latin mortalis also was used as a noun, "a man, mortal, human being."
mortal (adj.) Look up mortal at Dictionary.com
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."

This is reconstructed to be from PIE *mr-o- "to die," *mr-to- "dead," *mr-ti- "death" (sources also of Sanskrit mriyate "to kill," martave "to die," mrta- "died, dead," mrtih "death," martah "mortal man;" amrta- "immortal;" Avestan miriia- "to die," miryeite "dies," Old Persian martiya- "man;" Hittite mer- "to disappear, vanish," marnu- "to make disappear;" Armenian meranim "to die;" Greek emorten "died," brotos "mortal" (hence ambrotos "immortal"); Latin mori "to die;" Armenian merani- "to die;" Gothic maurþr, Old English morþ "murder;" Old Irish marb, Welsh marw "dead;" Lithuanian mirti "to die," mirtis "mortal man;" Old Church Slavonic mreti "to die," mrutvu "dead;" Russian mertvyj, Serbo-Croatian mrtav "dead").

The most widespread Indo-European root for "to die," forming the common word for it except in Greek and Germanic. Watkins says from PIE root *mer- (2) "to rub away, harm," with derivatives referring to death and human beings (source also of Sanskrit mrnati "crushes, bruises;" Greek marainein "to consume, exhaust, put out, quench," marasmus "consumption").