Fell from his greatness/ to be reduced in importance/ diminished in stature
He descended from the high status he had. He deteriorated in the social hierarchy.
("One who ascends to the roof in a dream ascends to greatness; one who descends, descends from his greatness" - Berakhot 57a, and also: "An incident occurred in which more than forty pairs passed by and Rabbi Akiva detained them... It is taught: Rabbi Judah said, Heaven forbid that Rabbi Akiva detained them. Rather, the fence at the head of the gate detained them, and Rabban Gamliel sent [someone] and had him removed from his greatness" - Rosh Hashanah 22a, and also: "And Joseph was brought down" - returning to the original subject, except that it interrupts it in order to connect Judah's descent to Joseph's sale, to tell you that because of him he was brought down from his greatness" - Rashi on Genesis 39:1)
Translation
Expression (Hebrew)
יָרַד מִגְּדֻולָּתוֹ
Definition (Hebrew)
יָרַד מֵהַמַּעֲמָד הַגָּבוֹהַּ שֶׁהָיָה לוֹ. הִדַּרְדֵּר בַּסֻּולָּם הַחֶבְרָתִי.
("הָעוֹלֶה לַגַּג בַּחֲלוֹם- עוֹלֶה לִגְדֻלָּה,יָרַד- יוֹרֵד מִגְּדֻלָּתוֹ"- בְּרָכוֹת נז ע"א, וְכֵן:"מַעֲשֶׂה שֶׁעָבְרוּ יוֹתֵר מֵאַרְבָּעִים זוּג וְעִכְּבָן רַבִּי עֲקִיבָא כו'. תָּנִיא, אָמַר רַבִּי יְהוּדָה: חַס וְשָׁלוֹם שֶׁרַבִּי עֲקִיבָא עִכְּבָן. אֶלָּא שֶׁזֶּפֶר- רֹאשָׁהּ שֶׁל גֶּדֶר- עִכְּבָן, וְשִׁלַּח רַבָּן גַּמְלִיאֵל וְהוֹרִידוּהוּ מִגְּדֻלָּתוֹ"- רֹאשׁ הַשָּׁנָה כב ע"א, וְכֵן:"וְיוֹסֵף הוּרַד- חוֹזֵר לָעִנְיָן רִאשׁוֹן, אֶלָּא שֶׁהִפְסִיק בּוֹ כְּדֵי לִסְמֹךְ יְרִידָתוֹ שֶׁל יְהוּדָה לִמְכִירָתוֹ שֶׁל יוֹסֵף, לוֹמַר לְךָ שֶׁבִּשְׁבִילוֹ הוֹרִידוּהוּ מִגְּדֻלָּתוֹ"- רָשִׁ"י בִּבְרֵאשִׁית לט א)