(Latin: based on castus "exempt (from mistakes), free, pure, chaste")
Etymologically, incest is actually the same word as unchaste. It was borrowed from Latin incestus, a noun use of an adjective formed from the negative prefix in- and castus, "pure".
The Latin word meant "unchastity" in general; however, in practice it was often applied specifically to "sexual contact between close relatives" as indicated in the word entries below all of which are completely the opposite of "purity" or "chastity" as indicated by the root word -cest for this unit.
2. Sexual relations between people 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.3. Etymology: from about 1225, "the crime of sexual intercourse between near kindred", from Latin incestum, "unchastity, not chaste, impure"; also specifically, "sex between close relatives"; from in-, "not" + castus, "pure".
In early use also with reference to "sexual relations" or "marriage with someone under a vow of chastity; sometimes distinguished as spiritual incest".
2. Having had a sexual relationship with someone who is considered to be too close as a relative.
3. Unhealthily intimate or interconnected; especially, so as to exclude the involvement or influence of others; an incestuous friendship.
4. Being so close or intimate as to prevent proper functioning: "There was an incestuous relationship between organized crime and government."
2. Sexual intercourse with one's daughter.