Well, the term is grossly over-exaggerated. If you were kicked by a mule, you could likely break a bone and be knocked down but even the 600 Nitro, the biggest bang for your buck, isn't capable of this.
And if you get kicked by a mule and are severely injured, you may never get to shoot again, so use the term while you still can!
I always was humored by the thought that when you fired a large gun, the stock would open and a spring loaded boot would launch out and KICK you, cartoon style.