HMM I KNOW ONE OF THESE ...

Definition :
You cannot see a snake in the grass until it is too late. If it is a dangerous snake it may attack you,bite you or hurt you in some way. That is what makes snakes so dangerous they cannot be seen. A person who is a snake in the grass is the same way. It is one thing to be a spiteful, back-stabbing individual. But if this person is understood to be such they can be avoided. A snake in the grass however does not let you know their intention, they will act as if they are your friend or pose no threat until it is too late and they suddenly attack you in some cruel way as to hurt you not necessarily physically but maybe emotionally or career wise even.