c.1200, "the crime of sexual intercourse between near kindred," from Old French inceste and directly from Latin incestum "unchastity, impious unchastity," also specifically "sex between close relatives," noun use of neuter adjective incestus "unchaste, impure," from in- "not" (see in-(1)) + castus "pure" (see caste). In early use also in reference to sexual relations or marriage with one under a vow of chastity (sometimes distinguished as spiritual incest). Old English had sibleger "incest," literally "kin-lying."