They went after vanity and became vain
They went after vain ideas and became fools. An expression mocking the throng’s following of vain ideas that influence their thinking (Hevel means breath or vapor, metaphorically: something without substance or use).
("Thus says the Lord, What evil have your fathers found in Me, that they have gone far from Me, and have walked after vanity, and are become vain?" - Jeremiah 2:5)
Translation
Expression (Hebrew)
וַיֵּלְכוּ אַחֲרֵי הַהֶבֶל וַיֶּהְבָּלוּ
Definition (Hebrew)
הָלְכוּ אַחֲרֵי רַעֲיוֹנוֹת שָׁוְא וְהָפְכוּ לְטִפְּשִׁים. הַבָּעַת לַעַג לִנְהִירַת הַהֲמוֹנִים אַחֲרֵי רַעֲיוֹנוֹת שָׁוְא הַמַּשְׁפִּיעִים עַל חֲשִׁיבָתָם (הֶבֶל מַשְׁמָע מַשַּׁב רוּחַ, בְּהַשְׁאָלָה: דָּבָר שֶׁאֵין בּוֹ שִׁמּוּשׁ).
("כֹּה אָמַר ה', מַה מָּצְאוּ אֲבוֹתֵיכֶם בִּי עָוֶל כִּי רָחֲקוּ מֵעָלָי וַיֵּלְכוּ אַחֲרֵי הַהֶבֶל וַיֶּהְבָּלוּ"- יִרְמְיָהוּ ב ה)