Etymology
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incubus (n.)
"imaginary being or demon, credited with causing nightmares, and, in male form, consorting with women in their sleep," c. 1200, from Late Latin incubus (Augustine), from Latin incubo "nightmare, one who lies down on (the sleeper)," from incubare "to lie upon" (see incubate). Plural is incubi. Compare succubus.
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succubus (n.)
late 14c., alteration (after incubus, giving a masc. form to a word generally felt as of female meaning) of Late Latin succuba "strumpet," applied to a fiend (generally in female form) having sexual connection with men in their sleep, from succubare "to lie under," from assimilated form of sub "under" (see sub-) + cubare "to lie down" (see cubicle). Related: Succubine (adj.).
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mare (n.3)

"night-goblin, incubus, oppressed sleep," Old English mare "incubus, nightmare, monster," from mera, mære, from Proto-Germanic *maron "goblin" (source also of Middle Low German mar, Middle Dutch mare, Old High German mara, German Mahr "incubus," Old Norse mara "nightmare, incubus"). This is from PIE *mora- "incubus" (source also of first element in Old Irish Morrigain "demoness of the corpses," literally "queen of the nightmare," also Bulgarian, Serbian mora, Czech mura, Polish zmora "incubus"). Also compare French cauchemar "nightmare," with first element from Old French caucher "to trample" and second element from Germanic.

All this is probably from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The word in English now survives only in nightmare (q.v.).

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nightmare (n.)

c. 1300, "an evil female spirit afflicting men (or horses) in their sleep with a feeling of suffocation," compounded from night + mare (n.3) "goblin that causes nightmares, incubus." The meaning shifted mid-16c. from the incubus to the suffocating sensation it causes. Sense of "any bad dream" is recorded by 1829; that of "very distressing experience" is from 1831.

Cognate with Middle Dutch nachtmare, German Nachtmahr. An Old English word for it was niht-genga. An 11c. gloss gives, for Latin Echo, Anglo-Saxon wudumær, a "wood-mere."

In the Anglo-Saxon superstitions, the Echo was supposed to be a spirit which dwelt in the wilds and mocked people who passed there, as the night-mare tormented people in bed. [Thomas Wright, "Anglo-Saxon and Old English Vocabularies," 1884]
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elf (n.)

"one of a race of powerful supernatural beings in Germanic folklore," Old English elf (Mercian, Kentish), ælf (Northumbrian), ylfe (plural, West Saxon) "sprite, fairy, goblin, incubus," from Proto-Germanic *albiz (source also of Old Saxon alf, Old Norse alfr, German alp "evil spirit, goblin, incubus"), origin unknown; according to Watkins, possibly from PIE *albho- "white." Used figuratively for "mischievous person" from 1550s.

In addition to elf/ælf (masc.), Old English had parallel form *elfen (fem.), the plural of which was *elfenna, -elfen, from Proto-Germanic *albinjo-. Both words survived into Middle English and were active there, the former as elf (with the vowel of the plural), plural elves, the latter as elven, West Midlands dialect alven (plural elvene).

The Germanic elf originally was dwarfish and malicious (compare elf-lock "knot in hair," Old English ælfadl "nightmare," ælfsogoða "hiccup," thought to be caused by elves); in the Middle Ages they were confused to some degree with faeries; the more noble version begins with Spenser. Nonetheless a popular component in Anglo-Saxon names, many of which survive as modern given names and surnames, such as Ælfræd "Elf-counsel" (Alfred), Ælfwine "Elf-friend" (Alvin), Ælfric "Elf-ruler" (Eldridge), also women's names such as Ælfflæd "Elf-beauty." Elf Lock hair tangled, especially by Queen Mab, "which it was not fortunate to disentangle" [according to Robert Nares' glossary of Shakespeare] is from 1592.

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goblin (n.)

early 14c., "a devil, incubus, mischievous and ugly fairy," from Norman French gobelin (12c., as Medieval Latin Gobelinus, the name of a spirit haunting the region of Evreux, in chronicle of Ordericus Vitalis), of uncertain origin; said to be unrelated to German kobold (see cobalt), or from Medieval Latin cabalus, from Greek kobalos "impudent rogue, knave," kobaloi "wicked spirits invoked by rogues," of unknown origin. Another suggestion is that it is a diminutive of the proper name Gobel.

Though French gobelin was not recorded until almost 250 years after appearance of the English term, it is mentioned in the Medieval Latin text of the 1100's, and few people who believed in folk magic used Medieval Latin. [Barnhart]
Thou schalt not drede of an arowe fliynge in the dai, of a gobelyn goynge in derknessis [Psalms xci.5 in the later Wycliffe Bible, late 14c.]
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demon (n.)
Origin and meaning of demon

c. 1200, "an evil spirit, malignant supernatural being, an incubus, a devil," from Latin daemon "spirit," from Greek daimōn "deity, divine power; lesser god; guiding spirit, tutelary deity" (sometimes including souls of the dead); "one's genius, lot, or fortune;" from PIE *dai-mon- "divider, provider" (of fortunes or destinies), from root *da- "to divide."

The malignant sense is because the Greek word was used (with daimonion) in Christian Greek translations and the Vulgate for "god of the heathen, heathen idol" and also for "unclean spirit." Jewish authors earlier had employed the Greek word in this sense, using it to render shedim "lords, idols" in the Septuagint, and Matthew viii.31 has daimones, translated as deofol in Old English, feend or deuil in Middle English. Another Old English word for this was hellcniht, literally "hell-knight."

The usual ancient Greek sense, "supernatural agent or intelligence lower than a god, ministering spirit" is attested in English from 1560s and is sometimes written daemon or daimon for purposes of distinction. Meaning "destructive or hideous person" is from 1610s; as "an evil agency personified" (rum, etc.) from 1712.

The Demon of Socrates (late 14c. in English) was a daimonion, a "divine principle or inward oracle." His accusers, and later the Church Fathers, however, represented this otherwise. The Demon Star (1895) is Algol (q.v.) .

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