Yew are
Gagsters - 2003 Dirty Rat was released on: USA: 2004
Although he never said it, James Cagney will forever be associated with the phrase "You dirty rat!"
Jimmy Cagney
Jimmy Cagney
Billy the Exterminator - 2009 Dirty Rat 4-8 was released on: USA: 23 August 2011
Kenan and Kel - 1996 You Dirty Rat 3-10 was released on: USA: 19 December 1998
Barney Miller - 1974 You Dirty Rat 2-9 was released on: USA: 13 November 1975
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A sewer or a big great dirty hole.
It means that he is despicable; he's done something horrible like betray someone. Rats are typically seen as "dirty" animals, so the allusion here is that he is like a rat and has no morals.
(Comets are said to be "dirty snowballs")