(1) [edge of knife]∶刀上用来切削的锋利的一边

(2) [knife-edge]∶像刀样锋利的窄棱或窄边(如天平或精密仪器上用以减少摩擦的轴上的)

(3) [the right spot]∶好的或恰当的地方

把劲儿使在刀口上

(4) [where it's needed most;the crucial point;right place where sth can be put to the best use]∶最需要的地方

钱要花在刀口上