let guard down
let (one's) guard down
To become less guarded or vigilant; to stop being cautious about potential trouble or danger. We'll wait long enough for them to let their guard down before we launch our invasion. There will always be people in this business looking to exploit you, so you can never let your guard down.
let one's guard down
Fig. to stop guarding oneself against trouble; to relax one's vigilance. He never lets his guard down because he trusts no one.