In the tough-talking, gin-soaked underworld of speakeasies, gun molls, hoods and coppers, a "rat fink" ranked amongst the lowest of the low. If a major crime boss such as Dutch Schultz or Al Capone ever saw the inside of a jail, it was most likely caused by the loose lips of a low-level associate trying to cut a deal. That unfortunate soul would be forever known as a rat fink, since he had the nerve to "rat out" his boss to the cops. An identified rat fink's life was often brutal and short.
In underworld crime parlance, being pegged as a fink was already a serious offense. A fink may crack under police pressure and name names or otherwise compromise a criminal operation, but he is also liable to spend some time in prison himself as an accomplice. A rat fink, on the other hand, may have ambitions of moving up in the ranks, so his information is leaked to law enforcement in a less obvious way. His reputation in the organization would still safe as long as no one connected him to the leak. A true rat fink usually lived out his criminal career in the murky middle, enjoying the benefits of the criminal lifestyle, but not getting too close to those he could betray.
There is another popular culture reference to a rat fink, and it has nothing to do with underworld gangsters. During the 1950s, an underground artist and custom hot rod car enthusiast named Ed "Big Daddy" Roth created a character based on his hatred of the sanitized Disney icon Mickey Mouse. Roth drew a rat, complete with bulging eyes, a mouthful of razor-sharp teeth and a decidedly nervous disposition. The rat character also wore a t-shirt with the initials "RF," which stood for "Rat Fink." Other artists went on to draw the character in comics and Roth's catalogs.
The Rat Fink character soon began to represent the burgeoning custom car and motorcycle building craze of the 1950s and 1960s, often referred to as Kustom Kulture. The Kustom Kulture phenomenon had a major influence on the hairstyles, clothing and attitude of the teens who grew up in that time period. "Rat Fink" was often portrayed as a freewheeling custom motorcycle rider, which helped to inspire such leather jacket and denim wearing characters as Fonzie on the television show Happy Days and greaser Danny Zuko in the musical Grease.