Read the dictionary definition of rat. All definitions for this word.
1. any of various long-tailed rodents similar to but larger than a mouse
2. a pad (usually made of hair) worn as part of a woman's coiffure
3. one who reveals confidential information in return for money
4. a person who is deemed to be despicable or contemptible
1. only a rotter would do that
2. kill the rat
3. throw the bum out
4. you cowardly little pukes!
5. the British call a contemptible person a `git'
5. someone who works (or provides workers) during a strike
6. give away information about somebody
1. He told on his classmate who had cheated on the exam
7. catch rats, especially with dogs
8. give (hair) the appearance of being fuller by using a rat
9. take the place of work of someone on strike
10. employ scabs or strike breakers in
11. desert one's party or group of friends, for example, for one's personal advantage
See also:
Merriam-Webster definition of rat