A "fin" is a slang term occasionally used for a $5 bill. The term "fin" has its origins in both the German and Yiddish languages, in which the word is loosely related to the word "five."
In addition to the term "fin," two other popular slang terms used for the $5 bill include "fiver" and "five-spot".
Slang terms for money are often in reference to the appearance, features, historical associations or the units of currency for the banknotes or coins involved. In the United States, some popular slang terms used for money in general include dough, smackers, simoleons and duckets.