Incest

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Middle English: from Latin incestus, incestum ‘unchastity, incest’, from in- ‘not’ + castus ‘chaste’.

wiktionary

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From Latin incestus.

etymonline

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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."