Etymology
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incest (n.)
"the crime of sexual intercourse between near kindred," c. 1200, from Old French inceste "incest; lechery, fornication," and directly from Latin incestum "unchastity, impious unchastity," also specifically "sexual intercourse between close relatives," noun use of neuter adjective incestus "unchaste, impure," from in- "not" (see in- (1)) + castus "pure" (see caste). Old English had sibleger "incest," literally "kin-lying."
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incestuous (adj.)
1530s, from Late Latin incestuosus "incestuous," from Latin incestus "unchaste" (see incest). Figurative use is from 1744. Related: Incestuously; incestuousness.
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*kes- 
Proto-Indo-European root meaning "to cut."

It forms all or part of: caret; cashier (v.) "dismiss;" cassation; caste; castellan; castellated; Castile; castle; castigate; castrate; castration; chaste; chastity; chateau; chatelaine; Chester; forecastle; incest; quash (v.) "make void, annul."

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sastra- "knife, dagger;" Greek keazein "to split;" Latin carere "to be cut off from," cassus "empty, void;" Old Church Slavonic kosa "scythe."
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abuse (v.)
Origin and meaning of abuse
early 15c., "to misuse, misapply" (power, money, etc.), from Old French abuser "deceive, abuse, misuse" (14c.), from Vulgar Latin *abusare, from Latin abusus "an abusing; a using up," past participle of abuti "use up, consume," also "misuse, abuse, misapply, outrage," from ab "off, away from" (see ab-) + uti "use" (see use).

Also in reference to forbidden sexual situations from early 15c., but originally meaning incest, masturbation (self-abuse), homosexuality, prostitution, etc. From 1550s specifically as "to misuse sexually, ravish," but OED 2nd ed. marks this obsolete and the modern use "subject (someone) to unwanted sexual activity" is likely a fresh coinage from late 20c. Specifically of drugs, from 1968. Meaning "attack with harsh language, revile" is from c. 1600. Related: Abused; abusing.
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homosexual (adj.)

1892, in C.G. Chaddock's translation of Krafft-Ebing's "Psychopathia Sexualis," from German homosexual, homosexuale (by 1880, in Gustav Jäger), from Greek homos "same" (see homo- (1)) + Latin-based sexual.

'Homosexual' is a barbarously hybrid word, and I claim no responsibility for it. It is, however, convenient, and now widely used. 'Homogenic' has been suggested as a substitute. [H. Havelock Ellis, "Studies in Psychology," 1897]

Sexual inversion (1883, later simply inversion, by 1895) was an earlier clinical term for "homosexuality" in English, said by Ellis to have originated in Italian psychology writing. See also uranian. Unnatural love was used 18c.-19c. for homosexuality as well as pederasty and incest. In 17c.-18c., pathic was used as a noun and adjective in reference to a man that submits to sexual intercourse with another man. Related: Homosexually.

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paraphilia (n.)

"sexual perversion, deviate desires," 1913, from German paraphilie (by 1903), apparently coined by Austrian ethnologist Friedrich Salomo Krauss (1859-1938) as meaning "inverted erotic instinct," from Greek para- "beside, aside" (see para- (1)) + philos "loving" (see -phile). Popularized in psychology circles in English from c. 1918 in translation of work by Viennese-born psychotherapist Wilhelm Stekel (1868-1940); not in widespread use until 1950s. It was added to the "Diagnostic and Statistical Manual of Mental Disorders" in 1980 as a morally neutral and more dignified label than perversion, to which it is nonetheless etymologically similar. Related: Paraphiliac; paraphilic.

The neurotic whose accompanying fancies always lead into forbidden ground (and this is what constitutes the guilt feeling of pollutions) fights against masturbation [pollutions] because it is connected with incest fancies, criminal desires, perversions, or as F.S. Krauss calls them, paraphilias. [Wm. J. Robinson, M.D., "Masturbation — Injurious or Harmless," "American Journal of Urology," May 1913]
Krauss bereichert uns um das neue Wort "Paraphilie" anstelle der "Psychopathie," ein fortschrittlich-oppositionelles Wort zwar, aber auch nur ein Wort und als Aufklärung etwa so bedeutsam wie "Seitensprünge." ["Rezensionen" über die "Anthropophyteia Jahrbücher," Leipzig, 1907]
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