Etymology
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mortal (adj.)

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.

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mortal (n.)

1520s, "mortal thing or substance;" 1560s, "a human being" (as subject to death); from mortal (adj.). Latin mortalis also was used as a noun, "a man, mortal, human being."

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ante-mortal (adj.)
"occurring before death," 1827; see ante- + mortal (adj.).
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mortally (adv.)
late 14c., "to the death; resulting in death," also "bitterly, intensely," from mortal (adj.) + -ly (2).
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rigor mortis 

"characteristic stiffening of the body caused by contraction of muscles after death," 1837, from Latin rigor "stiffness" (see rigor) + mortis, genitive of mors "death" (see mortal).

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mortmain (n.)

"inalienable ownership," mid-15c., from Anglo-French morte mayn (mid-14c.), Old French mortemain, literally "dead hand," from Medieval Latin mortua manus; for first element see mortal (adj.); second is from PIE root *man- (2) "hand." Probably a metaphorical expression on the notion of dead hands as those that cannot alienate.

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feuillemorte (adj.)
"of the color of a dead leaf," 1640s, fieulamort, from French feuille morte, literally "dead leaf" (see folio + mortal (adj.)). A word of loose spelling, variants include phyllamort, filemot, philomot.
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mortgage (n.)
Origin and meaning of mortgage

late 14c., morgage, "a conveyance of property on condition as security for a loan or agreement," from Old French morgage (13c.), mort gaige, literally "dead pledge" (replaced in modern French by hypothèque), from mort "dead" (see mortal (adj.)) + gage "pledge" (see wage (n.)).

So called because the deal dies either when the debt is paid or when payment fails. Old French mort is from Vulgar Latin *mortus "dead," from Latin mortuus, past participle of mori "to die" (from PIE root *mer- "to rub away, harm," also "to die" and forming words referring to death and to beings subject to death). The -t- was restored in Modern English based on Latin.

And it seemeth, that the cause why it is called mortgage is, for that it is doubtful whether the feoffor will pay at the day limited such sum or not: and if he doth not pay, then the land which is put in pledge upon condition for the payment of the money, is taken from him for ever, and so dead to him upon condition, &c. And if he doth pay the money, then the pledge is dead as to the tenant, &c. [Coke upon Littleton, 1664]
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*mer- 

Proto-Indo-European root meaning "to rub away, harm." Possibly identical with the root *mer- that means "to die" and forms words referring to death and to beings subject to death.

It forms all or part of: amaranth; ambrosia; amortize; Amritsar; immortal; manticore; marasmus; mare (n.3) "night-goblin, incubus;" morbid; mordacious; mordant; moribund; morsel; mort (n.2) "note sounded on a horn at the death of the quarry;" mortal; mortality; mortar; mortgage; mortify; mortmain; mortuary; murder; murrain; nightmare; post-mortem; remorse.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mrnati "crushes, bruises," 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 marainein "to consume, exhaust, put out, quench," marasmus "consumption," emorten "died," brotos "mortal" (hence ambrotos "immortal"); Latin mors (genitive mortis) "death," mori "to die;" Armenian merani- "to die;" Gothic maurþr, Old English morþ "murder;" Old Irish marb, Welsh marw "dead;" Lithuanian mirti "to die," mirtis "death;" Old Church Slavonic mreti "to die," mrutvu "dead;" Russian mertvyj, Serbo-Croatian mrtav "dead."

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mortality (n.)

mid-14c., mortalite, "condition of being subject to death or the necessity of dying," from Old French mortalite "massacre, slaughter; fatal illness; poverty; destruction" (12c.) and directly from Latin mortalitem (nominative mortalitas) "state of being mortal; subjection to death," from mortalis "subject to death, mortal," from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death).

Meaning "widespread death, numerousness of deaths; plague" is from c. 1400; meaning "number of deaths from some cause or in a given period" is from 1640s, later especially in proportion to population.

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