noun

1. any of numerous long-tailed rodents of various families (esp. Muridae and Cricetidae) resembling, but larger than, the mouse: rats are very destructive pests and carriers of highly contagious diseases, as bubonic plague and typhus
2. ☆ a small pad formerly used in certain styles of women's coiffures to make the hair look thicker
3. Slang a sneaky, contemptible person; specif.,
   1. an informer; stool pigeon
   2. a worker who is a scab
   3. a person who deserts or betrays a cause
4. Slang a person who spends a great deal of time at, in, or on a (specified) place a gym rat, a mall rat

Etymology: ME ratte < OE ræt, akin to Ger ratz, ratte < PGmc *ratto < ? IE base *red-, *rōd-, to scratch, gnaw > L radere, to scrape, rodere, to gnaw

intransitive verb ratted rat′·ted, ratting rat′·ting

1. to hunt for rats, esp. with dogs
2. Slang
   1. to desert or betray a cause, movement, etc. as rats are reputed to desert a sinking ship
   2. to act as a stool pigeon; inform (on)

transitive verb