חֵטְא חָטְאָה יְרוּשָׁלִַם, עַל-כֵּן לְנִידָה הָיָתָה; כָּל-מְכַבְּדֶיהָ הִזִּילוּהָ כִּי-רָאוּ עֶרְוָתָהּ, גַּם-הִיא נֶאֶנְחָה וַתָּשָׁב אָחוֹר.
Jerusalem hath grievously sinned, therefore she is become as one unclean; all that honoured her despise her, because they have seen her nakedness; she herself also sigheth, and turneth backward. (JPS)
The Hebrew word נידה (nidah) is rendered here “unclean.” The translator understands the word nidah to refer to a menstruant women, who is called nidah in Hebrew and is ritually “unclean.” However, the word for ritual impurity is טמא (tamé), so why is she called nidah? What does nidah mean?
The answer is in Rashi’s comments to the above verse. Rashi renders the word nidah here differently than the JPS translator. Rashi translates nidah here as “exiled.” Thus, the verse reads:
“Jerusalem hath grievously sinned, therefore she is become as one exiled…”
The word nidah, Rashi explains, is related to the word נד (nad) as in Genesis 4:12:
כִּי תַעֲבֹד אֶת-הָאֲדָמָה, לֹא-תֹסֵף תֵּת-כֹּחָהּ לָךְ; נָע וָנָד, תִּהְיֶה בָאָרֶץ.
When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a wanderer shalt thou be in the earth.’ (JPS)
Just as nad in Gen. 4:12 means “wanderer,” so too is the meaning here. Jerusalem went into exile, becoming a wanderer.
The reason this same word — nidah — is applied to someone ritually “unclean,” is because in that state, they must keep their distance from items that require ritual purity. Such a person likewise may not enter the Holy Temple. Nowadays, in the absence of the Holy Temple and its accompanying rituals, the only practical manifestation of this state of “impurity” is the physical contact that must be avoided between husband and wife until she can become purified from this state. Nevertheless, this “distance” that is kept is called a “wandering” or “exile” of sorts.
Rashi also uses an Old French word here to convey his definition of nidah: אישמוב”ר.
I believe this is related the Old French word “esmovoir” or one of the alternative forms “esmouvoir” or “esmoveir,” all of which mean, “to move.” The French antecedent to the English word “move” here is also obvious.
Ultimately, Rashi means to convey that nidah here means someone on the move, i.e. an exile or wanderer. This fits the verse in context nicely, without the necessity to refer to a concept related to ritual purity.
Nice moves, Rashi!