lower your guard
lower (one's) guard
To become less guarded or vigilant; to stop being cautious about potential trouble or danger. We'll wait long enough for them to lower their guard before we launch our invasion. There will always be people in this business looking to exploit you, so you can never lower your guard.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
lower your guardor
drop your guard
COMMON If you lower your guard or drop your guard, you stop taking actions to protect yourself against trouble and you start to relax, often with bad consequences. Note: Your guard is the position that you get in when you are ready to defend yourself in sports such as boxing or fencing. The soldiers had made the mistake of lowering their guard at the precise moment when they should have been most alert. I don't like to drop my guard when I'm with my colleagues, even if I'm socializing with them. Note: You can also say that you let your guard down or let down your guard with the same meaning. Many men work in extremely competitive environments where letting down their guard could leave them vulnerable.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
lower (or drop or let down) your guard1 relax your defensive posture, leaving yourself vulnerable to attack. 2 reduce your level of vigilance or caution.
This is an expression connected in its literal sense with boxing, as is its opposite raise your guard meaning ‘adopt a defensive posture’.
Farlex Partner Idioms Dictionary © Farlex 2017