Rat  —n. 1 a rodent like a large mouse. B similar rodent (muskrat; water-rat). 2 turncoat. 3 colloq. Unpleasant or treacherous person. 4 (in pl.) Slang exclamation of annoyance etc. —v. (-tt-) 1 hunt or kill rats. 2 (also foll. By on) inform (on); desert, betray. [old english]