To be so insanely tough and merciless. Your average street tough might be hard. But Arnold Schwarzenegger as John Matrix in his 80's hit "Commando", and John Rambo played by Sylvester Stallone are examples of "Hard as Fuck"
Col. John Matrix kills 200 people with an enormous machine gun with his pecs bouncing with recoil. He is hard as fuck.
Like in Dawn Of The Dead.Trapped on a bus surrounded by fucking zombies with nothing but a single bullet and a propane tank. You shoot the tank and blow yourself and all those fuckers straight to hell. Thats Hard As Fuck!