It's difficult to define a Finfag. You have to encounter one before you can truly understand their monkey-
like nature.
However, a Finfag (sometimes spelled finfag) is a
Finn (somebody who is from a country called Finland) whose IQ is lower than the number of his (Finfags are always males) shoe size.
Heaps of Finfags can be found in
Call of Duty: Modern Warfare 2.