Numbers 5:18

וְהֶעֱמִיד הַכֹּהֵן אֶת הָאִשָּׁה לִפְנֵי ה’ וּפָרַע אֶת רֹאשׁ הָאִשָּׁה וְנָתַן עַל כַּפֶּיהָ אֵת מִנְחַת הַזִּכָּרוֹן מִנְחַת קְנָאֹת הִוא וּבְיַד הַכֹּהֵן יִהְיוּ מֵי הַמָּרִים הַמְאָרְרִים

The priest shall stand the woman before Hashem and let the hair of the woman's head loose, and place the meal-offering of memorial in her hands, which is the meal-offering of jealousy, and the priest shall have in his hand the water of bitterness that causes the curse.

Most of the commentators understand the word m’ar’rim to mean that the bitter waters cause the curse, or perhaps reveal the curse. Yonasan ben Uziel, however, renders it “bitter water of investigation.” Presumably, he feels that m’ar’rim comes from the word “ohr” (light) and the waters “bring the matter to light.”