Leviticus 18:12

Thou shalt not uncover the nakedness of thy father's sister,.... His aunt by his father's side, an instance of which we have in Amram, Exodus 6:20; and Maimonides says i, an aunt was forbidden whether she was a father's sister in lawful wedlock or in fornication:

she [is] thy father's near kinswoman; or, the rest of thy father k; the residue of his flesh, one of the same flesh and blood with him; wherefore, as he could not marry her himself, so his son likewise was too near akin to enter into such a relation with her.

i Hilchot Issure Biah, c. 2. sect. 5. k שאר אביך "reliquiarum patris tui", Tigurine version.