c. 1400, "to die," from Old French expirer "expire, elapse" (12c.), from Latin expirare/exspirare "breathe out, blow out, exhale; breathe one's last, die," hence, figuratively, "expire, come to an end, cease," from ex "out" (see ex-) + spirare "to breathe" (see spirit (n.)). "Die" is the older sense in English; that of "breathe out" is attested from 1580s. Of laws, patents, treaties, etc., mid-15c. In 17c. also transitive. Related: Expired; expiring.
1730, "dead person," now mostly as a term in U.S. law, from Latin decedentem, present participle of decedere "to die, to depart" (see decease (n.)).
late 14c., "deadly, destructive to life; causing or threatening death" (of illness, poisons, wounds, etc.); also, of persons or the body, "doomed to die, subject to death;" from Old French mortel "destined to die; deserving of death" and directly 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," all 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.
"Subject to death," hence "human, of or pertaining to humans" (early 15c.). Also from late 14c. as "implacable, to be satisfied only by death" (of hatreds, enemies, etc.). Meaning "extreme, very great" is from late 14c. A mortal sin (early 15c., opposed to venial) is one that incurs the penalty of spiritual death.