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.
Finfag 1: "GO KRANE IT'S LSAT NCIE"
Finfag 2: "YES PLS I NED SUPSKIPPERS FOR MY YOUTOUB CHENEL!!1"