拿住刀靶ná zhù dāo bǎ

give sb a handle (against oneself); be at sb’s mercy
❍ 谁知越使钱越叫人～儿，越发来讹。(《红楼梦》891) Yet the more I gave him,the more I was at his mercy and the more he blackmailed me.

拿住刀靶

意谓抓住把柄。《红楼梦》六八：　谁知越使钱越被人拿住了刀靶，越发来讹。