He struck his heart
Figuratively: He felt remorse. His conscience rebuked him.
("And it came to pass afterward, that David's heart smote him, because he had cut off the wing that belonged to Saul" - 1 Samuel 24:5)
Translation
Expression (Hebrew)
הִכָּהוּ לִבּוֹ
Definition (Hebrew)
בִּמְלִיצָה: חָשׁ חֲרָטָה. יָסַר אוֹתוֹ מַצְפּוּנוֹ.
("וַיְהִי אחֲרֵי כֵן וַיַּךְ לֵב דָּוִד אֹתוֹ עַל אֲשֶׁר כָּרַת אֶת כָּנָף אֲשֶׁר לְשָׁאוּל"- שְׁמוּאֵל א כַּד ה)