전도서 10:3 해설: Rashi, Kohelet Rabbah & Tze'enah Ure'enah

And proclaims to all that he is a fool. By his walk and with his speech, everyone recognizes that he is a fool.⁵His actions make it so obvious, that it is as if he proclaims to all that he is a fool. (Ibn Ezra)

“Even when a fool walks on the way his heart is lacking; and he says to everyone that he is a fool” (Ecclesiastes 10:3).
“Even when a fool walks on the way…” – a fool believes that all the people are fools like him, but he does not know that he is a fool and all the people are wise.

“And also” [10:3]. Wherever the evildoer and the fool go, his heart fails him and he tells everyone that he is a fool. That is to say, he informs on himself that he is a fool by his excessive talk.¹⁴⁰Rashi, Ecclesiastes, 10:3.