Leviticus 20:18

18 If a man doeth fleshly knowing with a woman in the flowing of blood of the month, and showeth her filth(hood), and she openeth the well of her blood, both they shall be slain from the midst of their people. (If a man doeth fleshly knowing with a woman during the flowing out of the blood of the month, and uncovereth her nakedness, and she openeth the well of her blood to him, they both shall be cut off from the midst of their people.)

Leviticus 20:18 Meaning and Commentary

Leviticus 20:18

And if a man shall lie with a woman having her sickness
Her monthly courses, which make her weak and languid, which is forbidden, ( Leviticus 18:19 ) ; this is not to be understood of a man's lying with his wife ignorantly, when in such a condition, for this being the case, he was only unclean seven days, ( Leviticus 15:24 ) ; whereas this made him and her liable to cutting off, as in an after clause; but of his lying with her, knowing this to be the case with her, and of which she could not be ignorant, and therefore both liable to the same punishment; or else of any other man lying with her, or of any man lying with any woman, married or unmarried, being in such circumstances: and shall uncover her nakedness;
that is, have carnal knowledge of her: he hath discovered her fountain;
from whence her issue of blood flows: and she hath uncovered the fountain of her blood; freely and willingly, as Aben Ezra observes; for if she had been forced, he alone would have been cut off; but both these phrases put together show agreement in this matter, that they both had knowledge of her case, and both consented to commit the sin: and both of them shall be cut off from among their people;
by death, either by the hand of the civil magistrate, the case being known and proved, or else by the hand of God being known by mortality or the pestilence, as the Targum of Jonathan, or they should die childless; (See Gill on Leviticus 20:17).

Leviticus 20:18 In-Context

16 A woman that lieth under any beast, be she slain together with it; their blood be on them.

17 He that taketh his sister, his father's daughter, or his mother's daughter, and seeth her filth(hood), and she seeth the shame of her brother, they have wrought an unleaveful thing, both shall be slain in the sight of their people; for they showed together their filth(hood), and they shall bear together their wickedness. (He who taketh to bed his sister, his father's daughter, or his mother's daughter, and seeth her nakedness, and she seeth her brother's nakedness, they both have done an unlawful thing, and they both shall be cut off in the presence of their people; for they uncovered their nakedness to each other, and so they shall bear their wickedness together.)

18 If a man doeth fleshly knowing with a woman in the flowing of blood of the month, and showeth her filth(hood), and she openeth the well of her blood, both they shall be slain from the midst of their people. (If a man doeth fleshly knowing with a woman during the flowing out of the blood of the month, and uncovereth her nakedness, and she openeth the well of her blood to him, they both shall be cut off from the midst of their people.)

19 Thou shalt not discover the filth(hood) of thy mother's sister, and of thy father's sister; he, that doeth this, shall make naked the shame of his flesh, and both they shall bear their wickedness. (Thou shalt not uncover the nakedness of thy mother's sister, or of thy father's sister; he, who doeth this, shall make naked his own kin's flesh, and they both shall bear their wickedness.)

20 He that doeth fleshly knowing with the wife of his father's brother, either of his mother's brother, and showeth the filth(hood) of his kin, both they shall bear their sin, [and] they shall die without free children. (He who doeth fleshly knowing with the wife of his father's brother, or the wife of his mother's brother, and uncovereth the nakedness of his kin, they both shall bear their sin, and they shall die without any children.)