To throw stones at the fallen/to add insult to injury/ to hit a man when he is down
By metaphor: to continue harming a person who is already injured and collapsed. To add doing harm to a weak person or one already in distress (דָּחָה means push, impulse, the act of pushing).
("And the Tanna of the house of Rabbi Yishmael said: Since he went and sold himself to the service of stars, shall I push (throw a stone) after the fallen?" - Arachin 30b)
Translation
Expression (Hebrew)
דָּחָה אֶבֶן אַחֲרֵי הַנּוֹפֵל, לִדְחוֹת אֶבֶן אַחֲרֵי הַנּוֹפֵל
Definition (Hebrew)
בְּהַשְׁאָלָה: הִמְשִׁיךְ לִפְגֹּעַ בְּאָדָם שֶׁכְּבָר נִפְגַּע וְהִתְמוֹטֵט. לְהוֹסִיף לַעֲשׂוֹת רַע לְאָדָם חַלָּשׁ אוֹ שֶׁכְּבָר שָׁרוּי בְּצָרָה (דָּחָה מַשְׁמָע דַּחַף, הַדַּף).
("וְתַנָּא דְּבֵי רַבִּי יִשְׁמָעֵאל: הוֹאִיל וְהָלַךְ וּמָכַר עַצְמוֹ לַעֲבוֹדַת כּוֹכָבִים אִדָּחֶה (אֶדְחֹף) אֶבֶן אַחַר הַנּוֹפֵל?"-עֲרָכִין ל ע"ב )