Mishnayos Chagigah Perek 1 Mishnah 7

Rabbi Shimon ben Menasya says: Who is the crooked that cannot be made straight? This verse is referring to one who engaged in intercourse with a woman forbidden to him and fathered a mamzer with her. This individual is unable to rectify his sin, because the status of the illegitimate child is permanent. And if you say that it is referring to one who steals or robs, although he is crooked he can return what he stole and in this manner his sin will be rectified. Rabbi Shimon ben Yoḥai says: One calls crooked only someone who was initially straight and subsequently became crooked. And who is this? This is a Torah scholar who leaves his Torah study. Here is an example of something straight that became crooked.

רַבִּי שִׁמְעוֹן בֶּן מְנַסְיָא אוֹמֵר, אֵיזֶהוּ מְעֻוָּת שֶׁאֵינוֹ יָכוֹל לִתְקֹן, זֶה הַבָּא עַל הָעֶרְוָה וְהוֹלִיד מִמֶּנָּה מַמְזֵר. אִם תֹּאמַר בְּגוֹנֵב וְגוֹזֵל, יָכוֹל הוּא לְהַחֲזִירוֹ וִיתַקֵּן. רַבִּי שִׁמְעוֹן בֶּן יוֹחַאי אוֹמֵר, אֵין קוֹרִין מְעֻוָּת אֶלָּא לְמִי שֶׁהָיָה מְתֻקָּן בַּתְּחִלָּה וְנִתְעַוֵּת, וְאֵיזֶה, זֶה תַּלְמִיד חָכָם הַפּוֹרֵשׁ מִן הַתּוֹרָה: