Comment by Izkata

I believe it's a legal term that's leaked out into general use:

Wikipedia: https://en.wikipedia.org/wiki/Deadly_force

> Deadly force, also known as lethal force, is the use of force that is likely to cause serious bodily injury or death to another person.

Cornell Law School: https://www.law.cornell.edu/cfr/text/10/1047.7

> (a) Deadly force means that force which a reasonable person would consider likely to cause death or serious bodily harm.