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").
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."