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incest

/ˈɪn.sɛst/

Sexual relations between close relatives

From Latin in- (not) + Latin castus (pure).

noun
verb
in-
Latin
AI-inferred
in-
negative prefix: 'not, without'
Latin
Verified
incestus
formed with in- + castus, 'unchaste' or 'impure'

from Latin incestus. Displaced Old English mǣġhǣmed (literally “relative-sex”). Doublet of inchaste. ===...

castus
Latin
AI-inferred
castus
meaning 'pure, chaste'
Latin
Verified
incestus
the purity of castus negated

from Latin incestus. Displaced Old English mǣġhǣmed (literally “relative-sex”). Doublet of inchaste. ===...

Combined
incestus / incestum
A Latin formation meaning 'unchaste, impure'; later borrowed through Old French into Middle English as incest
Old French
Verified
inceste
used for incest, lechery, fornication

from Old French inceste "incest; lechery, fornication," and directly

Middle English
Verified
incest
borrowed c. 1200 into English legal and moral vocabulary

from Old French inceste "incest; lechery, fornication," and directly

+1 more source
Modern English
incest

This is one of those words where Latin does the moral arithmetic for you: take castus, “pure,” slap on in-, “not,” and you get incestus, the filthy opposite of the chaste ideal Romans prized. The same purity-idea gave English chaste, chastity, and even the social word caste, so the family resemblance is a little eerie: a whole cluster built around who belongs where and who stays clean. English didn’t invent a new taboo word from scratch; it borrowed Old French inceste around 1200, and the older Anglo-Saxons had their own blunt term, sibleger, literally “kin-lying.” That’s wonderfully raw compared with the polished Latin version, which sounds like it came from a senator’s desk instead of a kitchen table. By the time the word settled in English, it carried not just a legal accusation but a whole Roman worry about purity, inheritance, and the boundaries of the household — one tiny prefix turning “pure” into a social catastrophe.

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