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," sll from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The most widespread Indo-European root for "to die," it forms the common word for it except in Greek and Germanic.
"mortal thing or substance," 1520s, from mortal (adj.). Latin mortalis also was used as a noun, "a man, mortal, human being."