"Rocky, at least take this Pistol... – No thanks chief, don't need it!"
Fists of Fury is an unlockable passive Ability. It increases the Damage inflicted by Fists by +1. Additionally, by using one's fists, the Trooper has a 50% chance of disarming his target.
This Skill might be of use to troopers who often combine Initiative/Speed/Dodge skills, generally as a last resort for when all weapons have been affected by Sabotage or in situations when the trooper is out of ammo — for example, Exterminate missions.
Since Fists are generally ineffective, albeit used frequently, it would make sense to pick this upgrade to ensure somebody can still fight even without firearms. Despite being less effective than the Knife, fists can't be sabotaged at all.
The name "Fists of Fury" is likely referring to the 1972 film starring Bruce Lee, Fist of Fury . This is one of two Bruce Lee references in the game, the other being Wife Beater.
The quote, "Rocky, at least take this Pistol..." is a reference to the American film Rocky. This is one of two Rocky references, the other being Eye of the Tiger.
With Fists of Fury, you don't need to hit the arm to make the enemy drop his weapon. [Click to enlarge]