Leviticus 18:17

The nakedness of a woman and of her daughter thou shalt not dishonour, neither shalt thou take her son's daughter or the daughter of her daughter, to dishonour their nakedness; for they are of kin to her flesh; it is corruption.

kin [Hebrew: sha’ra] Hebrew contains two synonyms: sh’er [:6] and sha’ra . The ’alef in sha’ra would be vocalized with a schwa (like the lamed in “garment” [Hebrew: salma] [Exodus 22:8]) if it were not a guttural consonant.

IT IS ‘ZIMAH’ (LEWDNESS). Onkelos translated: “it is a sinful plan,” and Rashi commented thereon, “because your evil inclination counsels you to sin.” But I know of no sense to this, for with regard to all forbidden relations and all other transgressions, it is the evil inclination that counsels one to sin! [Why then is this verse prohibiting intercourse with a woman and her daughter, singled out by the use of this expression?] But the meaning of the matter is that the term zimah is derived from the expression, as ‘zamam’ (he had purposed) to do unto his brother. A proper thought is called m’zimah, as it said, And no ‘m’zimah’ (purpose) can be withholden from Thee, and lewdness that is thought of in secret is called zimah, similar to that which it is said, If my heart have been enticed unto a woman,, 31:9. and Scripture concludes, For that were ‘zimah’ (a heinous crime); yea, it were an iniquity to be punished by the judges., Verse 11. Similarly, ‘v’zimatheich’ (thy lewdness) and thy harlotries; and ‘zimath’ (the lewdness of) thy harlotry, denoting these illicit acts committed in private and in public. In my opinion Scripture states it is ‘zimah’, in the case of [sexual relationships with] a woman and her daughter, and also a woman and her mother, in order to condemn the matter, saying that when one lies with the one, who is his wife, he thinks of the other one on account of their relationship and likeness, and thus lying with both of them is a cause of lewdness to him. This matter is similar to that which the Rabbis have mentioned: “And I will purge out from among you the rebels, and them that transgress against Me. These are the children [who are the offspring] of nine different dispositions [in their parents] etc.” It is for this reason that Scripture states here, the nakedness of a woman and her daughter etc., and similarly it says, And if a man take with his wife also her mother, it is lewdness, and does not say “the nakedness of your wife’s daughter, or your wife’s mother, you shalt not uncover” [but rather mentions them in each case together, by saying: the nakedness of a woman and her mother, or the nakedness of a woman and her daughter, in order to indicate that the prohibition is on account of thinking of the two together]. And the explanation of the verses here is …

זימה היא, it is sexual union based only on lust and sinful intent.