A rat is an animal which has a long tail and looks like a large mouse.

If you call someone a rat, you mean that you are angry with them or dislike them, often because they have cheated you or betrayed you.

If someone rats on you, they tell someone in authority about things that you have done, especially bad things.

If someone rats on an agreement, they do not do what they said they would do.

If you smell a rat, you begin to suspect or realize that something is wrong in a particular situation, for example that someone is trying to deceive you or harm you.