expression, godan verb (-su)
let one's guard down; drop one's guard; be off one's guard
Used when someone relaxes their vigilance or caution, often in a social or emotional context, making themselves vulnerable.
彼は親友にだけは気を許している。
He lets his guard down only with his close friends.
知らない人に気を許
You mustn't let your guard down with strangers.