警けい戒かい心しんを解とく

expression, godan verb (-ku)

let down one's guard; relax one's guard

Used when someone becomes less cautious or vigilant, often after feeling safe or comfortable.

• 彼かれは親したしくなるとすぐに警けい戒かい心しんを解とく。

  He quickly lets down his guard once he gets to know someone.

• 相あい手てが笑え顔がおを見みせたので、警けい戒かい心しんを解といてしまった。

  Because the other person smiled, I let my guard down.