[luò jǐng tóu shí]   

落    井    投    石    比喻趁人有危难时加以陷害。

成语解释比喻趁人有危难时加以陷害成语出处沙汀《炮手》:"从未做过落井投石的事。"

Take mean advantage of someone when he is down

落阱下石    落拓不羁    落花无言    落草为寇    落魄不偶