A soldier or actual civilian that is hired by private sources to do the "dirty work" for others who are too scared or skillfully lacking to do it themselves... other terms are Mercenary, Dog of War and Pawn. You remember the game Metal Gear Solid? ...well Solid Snake was a Soldier of Fortune, so if you remember what the story was about, you'll find out exactly what a Soldier of Fortune does. They are also kept top secret and thier existance is denied and all records of them of everything they've ever done gets destroyed, because what they go out to do, they hardly ever come back because they generally have nothing to lose.
Private miller: oh man! last night on watch, I saw a man dressed in military uniform stalking alone in the middle of the jungle! He was all alone and his uniform didn't look like any army i'v seen before... he killed one of our look outs without any of us knowing, he must have been so skilled and quiet to get away with that!
Colonel Hemmel: Holy shit! We got ourselves a Soldier of Fortune on our hands!