1. One who is regarded as a traitor for reporting criminal activity to the police. Primarily heard in US. The gang was determined to find the rat fink who told the police about their illegal activities.
2. An unpleasant person. Mike is such a rat fink that I can't even stand being in the same room as him.
an extremely unpleasant person, or someone who has given secret information about you to the police If I find the rat fink who informed on me, he won't live long enough to do it again.