Radak citing Targum Yonasan: He risked his life like on the back to be killed. Lev Yitzchak (Shoftim 12:3) - some texts of Targum Yonasan [there] say "k'Al Gav Yadi", like the Targum in Tehilim 119:109 and Iyov 13:14. I.e. my life was prone to be lost like something on back of the hand is prone to be dropped.