Webster's English Dictionary

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rat

1. rat \'rat\ n [ME, fr. OE rt; akin to OHG ratta rat, L rodere to 
   g]naw, radere to scrape, shave 1a: any of numerous rodents (Rattus and 
   related genera) differing fro m the related mice by considerably larger 
   size and by features of the teeth and other structures 1b: any of various 
   similar rodents  2: a contemptible person : as  2a: BETRAYER  2b: SCAB  2c: 
   INFORMER  3: a pad over which a woman's hair is arranged 
2. rat vi or rat.ted;  or rat.ting 1: to desert or inform on one's 
   associates  2: to catch or hunt rats  3: to work as a scab  4: to reverse 
   one's position : RECANT