Rat-fink

Doesn't the word rat-fink send an image of greasy man in a shady pawn shop who has his hair slicked back? I definitely get this image when I hear this word.

According to the AHD a rat-fink is a slang word that is:

1. A contemptible, obnoxious, or otherwise undesirable person.

2. An informer.

The etymology of rat by itself ultimately comes from Old English, raet, and fink's etymology is ultimately unknown. It is another slang word meaning the same thing as rat-fink. So I'm guessing that you're doubly shady if you're a rat-fink, instead of just a rat or just a fink.