We have the usage of "rat" as an informer (grass, I believe it's called in the UK), which, in recent years has become a verb (see recent threads about verbing nouns), as in "to rat someone out." My father in law used to combine this with "fink" (meaning the same thing, but a word derived from labor unions) to make "ratfink". (Fink has also been verbed, as in "to fink on someone). My, my -- who would fink it?