It refers to the policy of not allowing any mercy or surrender to the occupants of a fortification once the battering ram begins the assault on the gates. This policy was to act as a deterrent against resistance to those about to be besieged. It was an incentive for anyone who was not absolutely sure that they could withstand the assault to surrender immediately, rather than face the possibility of total destruction.
I am delighted to discover that a mindset I try to cultivate can be expressed so neatly in Latin.
Also: that would look great on a business card.
From this letter, via Patrick at PopeHat.