Lower (one's) guard verb phrase common
Used to say that someone becomes less guarded or vigilant or ceases being wary of potential problems or danger.
He hit me while I was lowering my guard.
When he lowers his guard, you take this opportunity to kill him.
If you lower your guard, you will be dangerous.
To behave or act more carefully than one did before.
Protect one from harm caused by unforeseen events
To be cautious in what one says.
To keep watch or act a guard for someone or something.
If you ask someone to be on the watch for (someone or something), you mean that he has to look out for someone or something carefully and actively.
The verb "lower" should be conjugated according to its tense.