Etymology
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morbid (adj.)

1650s, "of the nature of a disease, indicative of a disease," from Latin morbidus "diseased," from morbus "sickness, disease, ailment, illness," according to de Vaan perhaps connected to the root of mori "to die," as "looking like death" (from PIE root *mer- "to rub away, harm," also "to die" and forming words referring to death and to beings subject to death), or from a non-IE word. Meaning "diseased, sickly" is from 1731. Transferred use, of mental states, "unwholesome, excessive, unreasonable" is by 1834. Related: Morbidly; morbidness. Middle English had morbous "diseased" (early 15c.), from Latin morbosus.

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morbidity (n.)

"morbid condition or state," 1721, from morbid + -ity or from French morbidité.

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premorbid (adj.)

also pre-morbid, "preceding the occurrence of symptoms or disease," 1905, from pre- "before" + morbid.

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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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nicotinism 

"morbid effects of excessive use of tobacco," by 1873, from nicotine + -ism.

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pyrophobia (n.)

"morbid fear of fire," 1871, from pyro- "fire" + -phobia "fear."

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phobophobia (n.)
"morbid dread of being alarmed," 1890; see phobia.
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aerophobia (n.)
"morbid dread of a current of air," 1785; see aero- + phobia.
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arachnophobia (n.)
"morbid fear of spiders," 1925, from combining form of arachnid + -phobia "fear."
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swelling (n.)
"tumor, morbid enlargement," Old English; verbal noun from swell (v.).
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