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."
incest in·cest (ĭn'sěst')
Sexual relations between persons who are so closely related that their marriage is illegal or forbidden by custom.
The statutory crime of sexual relations with such a near relative.
Sexual relations between relatives who are forbidden by law to marry; for example, between father and daughter or mother and son.
Note: Though each society has its own system for determining the range of people who fall into this category, every society has an incest taboo of some sort.