Chapter 15 of Leviticus serves as the basis for the Jewish menstrual laws. The Hebrew term used for menstruation in Leviticus 15:19, 20, 24, and 33 is niddatah, which has as its root ndh, a word meaning “separation,” usually as a result of impurity. It is connected to the root ndd, meaning “to make distant.” This primary meaning of the root was extended in the biblical corpus to include concepts of sin and impurity. The Aramaic Bible translations (Onkelos [second century c.e.], Pseudo-Jonathan and Neofiti) translate these verses with the root rhq, “in her separation/distance,” some adding “of her impurity.” Both roots reflect the physical separation of women during menstruation (or abnormal uterine bleeding or the seven or fourteen days immediately postpartum) from physical contact or from certain activities in which they would normally engage at other times. In other parts of the Bible, the term niddah was transferred to include abominable acts, objects (Ezekiel 7:19–20) or status, especially sexual sins (Leviticus 20:21) and idolatry. The use of the term niddah to describe the impurity of the land due to sin is found in Lamentations 1:8 and Ezra 9:11 and as an antonym of holiness in 2 Chronicles 29:5. These usages of the term may have influenced subsequent reactions to the state of menstruation. The term niddah was transformed into a metaphorical expression for sin and impurity in general. These meanings added to the original sense of distancing or separation, creating a new semantic range which influenced the legal and emotional understanding of niddah over the course of generations.
http://jwa.org/encyclopedia/article/female-purity-niddah