Reference for Niddah 2:5

מָשָׁל מָשְׁלוּ חֲכָמִים בָּאִשָּׁה, הַחֶדֶר וְהַפְּרוֹזְדוֹר וְהָעֲלִיָּה. דַּם הַחֶדֶר, טָמֵא. נִמְצָא בַפְּרוֹזְדוֹר, סְפֵקוֹ טָמֵא, לְפִי שֶׁחֶזְקָתוֹ מִן הַמָּקוֹר:

The Sages analogized women through a parable: a chamber, a corridor, and an upper chamber. The blood of the chamber is impure. If [blood is] found in the corridor, its uncertainty is impure [i.e. there is an uncertainty regarding its origin, and it is thereby impure], because it is presumed to have come from the source [i.e. from the chamber].

Jastrow

Jastrow