"You Dirty Rat!"
This is a very famous line from a movie from around my parents' era, so forgive me for not being certain of the origin, but I believe it was James Cagney, in one of his many "tough guy" roles in the early movies.
Now this was a pretty disparaging insult that was thrown at whoever
it was Mr. Cagney was sparring with - to equate someone with something
as low and vile as a Rat. Was it deserved? Do rats really belong near
the bottom when it comes to desirable things we want around us? Well,
possibly, but it takes some understanding of "rats".