[chéng xū dǎo xì]   

乘    虚    蹈    隙    犹言切中窾窍。比喻叩击中节。

犹言切中窾窍。比喻叩击中节。

Take advantage of sb.'s undefended state to attack

乘肥衣轻    乘车戴笠    乘云行泥    乘机打刼    乘龙佳壻