Etymology
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derangement (n.)

1737, "disturbance of regular order," from French dérangement (17c.), from déranger (see derange). Mental sense "disturbance of the intellect or reason" is from 1800.

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psychosis (n.)

1847, "mental affection or derangement," Modern Latin, from Greek psykhē "mind, life, soul" (see psyche) + -osis "abnormal condition." Greek psykhosis meant "a giving of life; animation; principle of life."

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psychopathy (n.)

1847, "derangement of the mental functions," from psycho- + -pathy, on the model of German Psychopathie. First attested in a translation of Feuchtersleben's "Lehrbuch der ärztlichen Seelenkunde" (1845). By 1891 as "cure of sickness by psychic influence or means" (hypnotism, etc.).

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alienist (n.)

"one who scientifically treats or studies mental illness," 1864, from French aliéniste, from alienation in the sense of "insanity, loss of mental faculty," from Latin alienare "deprive of reason, drive mad," literally "to make another's, estrange" (see alienate). The mental sense of alienare has since mostly died out in English, but Middle English had aliened from mind "deranged, not rational" (late 14c.), and alienation was used from 15c. in a sense of "loss or derangement of mental faculties, insanity."

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neurosis (n.)

1776, "functional derangement arising from disorders of the nervous system (not caused by a lesion or injury)," coined by Scottish physician William Cullen (1710-1790) from Greek neuron "nerve" (see neuro-) + Modern Latin -osis "abnormal condition." Originally of epilepsy, hysteria, neuralgia, etc. Used in a general psychological sense from 1871, "change in the nerve cells of the brain resulting in symptoms of stress," but not radical loss of touch with reality (psychosis); clinical use in psychiatry dates from 1923.

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paranoia (n.)

"mental disorder characterized by systematized delusions of more or less definite scope," 1848 (earlier paranoea 1811), from Greek paranoia "mental derangement, madness," from paranoos "mentally ill, insane," from para- "beside, beyond" (see para- (1)) + noos "mind," which is of uncertain origin.

FOR several years frequent descriptions have been given in the foreign journals, especially German and Italian, of the forms of insanity designated by the names Paranoia, Verrücktkeit, and Wahnsinn. ["Paranoia — Systematized Delusions and Mental Degenerations," J. Séglas (transl. William Noyes), 1888]
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delusion (n.)

"act of misleading someone, deception, deceit," early 15c., delusioun, from Latin delusionem (nominative delusio) "a deceiving," noun of action from past-participle stem of deludere (see delude). As a form of mental derangement, "false impression or belief of a fixed nature," 1550s.

Technically, delusion is a belief that, though false, has been surrendered to and accepted by the whole mind as a truth; illusion is an impression that, though false, is entertained provisionally on the recommendation of the senses or the imagination, but awaits full acceptance and may not influence action. Delusions of grandeur, the exact phrase, is recorded from 1840, though the two words were in close association for some time before that.

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malapropism (n.)

"act or habit of misapplying words through ambition to use fine language," also a word so misapplied, 1826, from Mrs. Malaprop, character in Sheridan's play "The Rivals" (1775), noted for her ridiculous misuse of large words (such as contagious countries for contiguous countries), her name coined from malapropos.

When Mrs. Malaprop, in Sheridan's Rivals, is said to 'deck her dull chat with hard words which she don't understand,' she protests, 'Sure, if I reprehend anything in this world it is the use of my oracular tongue, & a nice derangement of epitaphs'—having vague memories of apprehend, vernacular, arrangement, & epithets. She is now the matron saint of those who go wordfowling with a blunderbuss. [Fowler]
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mania (n.)

late 14c., "mental derangement characterized by excitement and delusion," from Late Latin mania "insanity, madness," from Greek mania "madness, frenzy; enthusiasm, inspired frenzy; mad passion, fury," related to mainesthai "to rage, go mad," mantis "seer," menos "passion, spirit," all of uncertain origin, perhaps from PIE *mnyo-, suffixed form of root *men- (1) "to think," with derivatives referring to qualities and states of mind or thought.

Mania is manifested by psychic elevation, increased motor activity, rapid speech and the quick flight of ideas. [Scientific American, September 1973]

Sense of "fad, craze, enthusiasm resembling mania, eager or uncontrollable desire" is by 1680s, from French manie in this sense. Sometimes nativized in Middle English as manye. Used since 1500s as the second element in compounds expressing particular types of madness (such as nymphomania, 1775; kleptomania, 1830; megalomania, 1890), originally in Medical Latin, in imitation of Greek, which had a few such compounds, mostly post-classical: gynaikomania (women), hippomania (horses), etc.

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legion (n.)

c. 1200, "a Roman legion," from Old French legion "squad, band, company, Roman legion," from Latin legionem (nominative legio) "Roman legion, body of soldiers, a levy of troops," from legere "to gather; to choose, pick out, select," from PIE root *leg- (1) "to collect, gather." Tucker writes that "The common sense is 'pick,'" but it is unclear whether the use here is "picking up or picking out." Roughly 3,000 to 6,000 men, under Marius usually with attached cavalry. "The legions were numbered in the order of their levy, but were often known by particular names" [Lewis].

The great power of the Roman legion was due to its rigid discipline and its tactical formation in battle, which was so open and flexible as to enable it to meet every emergency without surprise or derangement.

Generalized sense of "a large number of persons" (c. 1300) is due to translations of the allusive phrase in Mark v.9. Of modern military bodies from 1590s. American Legion, U.S. association of ex-servicemen, founded in 1919. Legion of Honor is French légion d'honneur, an order of distinction founded by Napoleon in 1802. Foreign Legion is French légion étrangère "body of foreign volunteers in a modern army," originally Polish, Belgian, etc. units in French army; they traditionally served in colonies or distant expeditions. Related: Legionary.

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