1. [n]  any of various long-tailed rodents similar to but larger than a mouse
2. [n]  one who reveals confidential information in return for money
3. [n]  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!"
4. [n]  someone who works (or provides workers) during a strike
5. [v]  give away information about somebody; "He told on his classmate who had cheated on the exam"
6. [v]  catch rats, esp. with dogs
7. [v]  give (hair) the appearance of being fuller by using a rat
8. [v]  take the place of work of someone on strike
9. [v]  employ scabs or strike breakers in (an industry)
10. [v]  desert one's party or group of friends, for example, for one's personal advantage

RAT is a 3 letter word that starts with R.

1. \Rat\, n. [AS. r[ae]t; akin to D. rat, OHG. rato, ratta, G.
   ratte, ratze, OLG. ratta, LG. & Dan. rotte, Sw. r[*a]tta, F.
   rat, Ir. & Gael radan, Armor. raz, of unknown origin. Cf.
   {Raccoon}.]
   1. (Zo["o]l.) One of the several species of small rodents of
      the genus {Mus} and allied genera, larger than mice, that
      infest houses, stores, and ships, especially the Norway,
      or brown, rat ({M. Alexandrinus}). These were introduced
      into Anerica from the Old World.
   
   2. A round and tapering mass of hair, or similar material,
      used by women to support the puffs and rolls of their
      natural hair. [Local, U.S.]
   
   3. One who deserts his party or associates; hence, in the
      trades, one who works for lower wages than those
      prescribed by a trades union. [Cant]
   
   Note: ``It so chanced that, not long after the accession of
         the house of Hanover, some of the brown, that is the
         German or Norway, rats, were first brought over to this
         country (in some timber as is said); and being much
         stronger than the black, or, till then, the common,
         rats, they in many places quite extirpated the latter.
         The word (both the noun and the verb to rat) was first,
         as we have seen, leveled at the converts to the
         government of George the First, but has by degrees
         obtained a wide meaning, and come to be applied to any
         sudden and mercenary change in politics.'' --Lord
         Mahon.
   
   {Bamboo rat} (Zo["o]l.), any Indian rodent of the genus
      {Rhizomys}.
   
   {Beaver rat}, {Coast rat}. (Zo["o]l.) See under {Beaver} and
      {Coast}.
   
   {Blind rat} (Zo["o]l.), the mole rat.
   
   {Cotton rat} (Zo["o]l.), a long-haired rat ({Sigmodon
      hispidus}), native of the Southern United States and
      Mexico. It makes its nest of cotton and is often injurious
      to the crop.
   
   {Ground rat}. See {Ground Pig}, under {Ground}.
   
   {Hedgehog rat}. See under {Hedgehog}.
   
   {Kangaroo rat} (Zo["o]l.), the potoroo.
   
   {Norway rat} (Zo["o]l.), the common brown rat. See {Rat}.
   
   {Pouched rat}. (Zo["o]l.)
      (a) See {Pocket Gopher}, under {Pocket}.
      (b) Any African rodent of the genus {Cricetomys}.
   
   {Rat Indians} (Ethnol.), a tribe of Indians dwelling near
      Fort Ukon, Alaska. They belong to Athabascan stock.
   
   {Rat mole}. (Zo["o]l.) See {Mole rat}, under {Mole}.
   
   {Rat pit}, an inclosed space into which rats are put to be
      killed by a dog for sport.
   
   {Rat snake} (Zo["o]l.), a large colubrine snake ({Ptyas
      mucosus}) very common in India and Ceylon. It enters
      dwellings, and destroys rats, chickens, etc.
   
   {Spiny rat} (Zo["o]l.), any South America rodent of the genus
      {Echinomys}.
   
   {To smell a rat}. See under {Smell}.
   
   {Wood rat} (Zo["o]l.), any American rat of the genus
      {Neotoma}, especially {N. Floridana}, common in the
      Southern United States. Its feet and belly are white.
   
   

2. \Rat\, v. i. [imp. & p. p. {Ratted}; p. pr. & vb. n.
   {Ratting}.]
   1. In English politics, to desert one's party from interested
      motives; to forsake one's associates for one's own
      advantage; in the trades, to work for less wages, or on
      other conditions, than those established by a trades
      union.
   
            Coleridge . . . incurred the reproach of having
            ratted, solely by his inability to follow the
            friends of his early days.            --De Quincey.
   
   2. To catch or kill rats.