rat 
(răt)
(răt)n.
1.
a. Any of various long-tailed rodents resembling mice but larger, especially one of the genus Rattus.
b. Any of various animals similar to one of these long-tailed rodents.
2. Slang
a. A despicable person, especially one who betrays or informs upon associates.
b. A scab laborer.
3. A pad of material, typically hair, worn as part of a woman's coiffure to puff out her own hair.
4. Slang A person who frequently passes time at a particular place. Often used in combination: a rink rat.
v. rat·ted, rat·ting, rats
v.intr.
1. To hunt for or catch rats, especially with the aid of dogs.
2. Slang To reveal incriminating or embarrassing information about someone, especially to a person in authority: ratted on his best friend to the police.
3. Slang To work as a scab laborer.
v.tr.
Phrasal Verb: To puff out (the hair) with or as if with a pad of material.
rat out
Slang To reveal incriminating or embarrassing information about (someone), especially to a person in authority: ratted out the vandals to the school principal.
[Middle English, from Old English ræt; see rēd- in the Appendix of Indo-European roots.]