We use this idiom to refer to people who pretend to be your friend but in fact they’re just present to cause you harm.
For example, a person you thought was your friend was just using you to get some kind of information, so we could say that your friend was a snake in the grass.
The idiom ‘Snake in the grass’ is used to refer to a sneaky and despised person. For example: Did you hear that Daria’s best friend stole money from her bankaccount? What a snake in the grass.
You can use the following website to search the meaning behind the idioms:
It is a great website where you just have to enter the idiom in the search bar to get the meaning of the idiom with examples of the same as well.
All the best for your future endeavours!