Unstoppable Force

 is a perk in Fallout: New Vegas.

Effects
"'Your martial might is truly legendary. You do a large amount of additional damage through enemy blocks with all Melee Weapons and Unarmed attacks.' -- In-Game Description"

More specifically, this perk multiplies your Melee and Unarmed damage by 4 when you strike a blocking opponent, which allows you to inflict serious damage in spite of the additional Damage Threshold provided by their blocking.

Maybe a reference to World of Warcraft, there is an LV60 epic battleground two-hand mace named "Unstoppable Force".