The Gemara Berachos (31b) writes
אֵלֵךְ וְאֶסְתַּתֵּר בִּפְנֵי אֶלְקָנָה בַּעֲלִי, וְכֵיוָן דְּמִסְתַּתַּרְנָא מַשְׁקוּ לִי מֵי סוֹטָה, וְאִי אַתָּה עוֹשֶׂה תּוֹרָתְךָ פְּלַסְתֵּר, שֶׁנֶּאֱמַר ״וְנִקְּתָה וְנִזְרְעָה זָרַע״.
What was Hannah threatening? She said:
I will go and seclude myself with another man before Elkana, my husband. Since I secluded myself, they will force me to drink the sota water to determine whether or not I have committed adultery. I will be found innocent, and since You will not make Your Torah false [pelaster], I will bear children. With regards to a woman who is falsely suspected of adultery and drank the sota water, the Torah says: “And if the woman was not defiled, but was pure, then she shall be acquitted and she shall conceive” (Numbers 5:28).
Though Jastrow's dictionary points to the definition of forgery, what is the origin or root of the word פְּלַסְתֵּר?