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.
See also: guard, lower
Farlex Dictionary of Idioms.

lower your guard

or

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.
See also: guard, lower
Collins COBUILD Idioms Dictionary, 3rd ed.

lower (or drop or let down) your guard

1 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’.
See also: guard, lower
Farlex Partner Idioms Dictionary
See also:
  • lower (one's) guard
  • drop (one's) guard
  • leave (one's) guard down
  • let (one's) guard down
  • let down (one's) guard
  • let guard down
  • be off (one's) guard
  • be on/off your guard
  • on (one's) guard
  • on guard
References in periodicals archive
It shows all them years his dad tried show people it's ok to smile and to lower your guard, it's ok to show love to others, finally happened.
Parklife founder's incredible gesture for late Piccadilly Rat Ray Boddington
The women, referred to in the entertainment joints in the coastal resort town as "swaras", he adds, will pretend to be really good company at your table but, as you lower your guard, they will spike your drink.
THE CUTTING EDGE
TUNES By John Dingwall Singles TeenCanteen You're Still Mine Our fave A BREATH of fresh air guitar-pop from the Scots all-girl quartet that will lower your guard from first beat until the last.
TUNES
"The moment you think you are too good for this division, you are going to get punished as soon as you lower your guard."
MONEY BAGGS; West Brom boss wants to say buy, buy to the Championship
Just like Spawn, if you're bigger than him and you lower your guard, the sucker punch will come right on in.
Men at work: etnies down under