rat in WordNet English dictionary

1. give away information about somebody
   "He told on his classmate who had cheated on the exam"
   denounce  tell on  betray  give away  rat  grass  shit  shop  snitch  stag 
   
2. take the place of work of someone on strike
   fink  scab  rat  blackleg 
   
3. catch rats, especially with dogs
   rat 
   
4. give (hair) the appearance of being fuller by using a rat
   rat 
   
5. desert one's party or group of friends, for example, for one's personal advantage
   rat 
   
6. employ scabs or strike breakers in
   rat 
   

1. one who reveals confidential information in return for money
   informer  betrayer  rat  squealer  blabber 
   
2. someone who works (or provides workers) during a strike
   scab  strikebreaker  blackleg  rat 
   
3. a person who is deemed to be despicable or contemptible
   "only a rotter would do that"; "kill the rat"; "throw the bum out"; "you cowardly little pukes!"; "the British call a contemptible person a `git'"
   rotter  dirty dog  rat  skunk  stinker  stinkpot  bum  puke  crumb  lowlife  scum bag  so-and-so  git 
   
4. any of various long-tailed rodents similar to but larger than a mouse
   rat 
   
5. a pad (usually made of hair) worn as part of a woman's coiffure
   rat