rat

(noun) someone who tells the authorities about the wrong or illegal actions of another.

Example Sentence:

Don’t tell the teacher that I cheated on the test, you rat.

I don’t want to be a rat, but I saw your girlfriend with another man last night.

Similar idioms:

1. rat on someone
2. rat someone out
3. man up
4. tell on someone
5. beside oneself
6. rug rat