I think that it's one of those things that has been over worked, over used and worn out because folks like the sound of it. It's been applied to everything from reassurance in awkward social settings where the only thing that's killed is someone's pride OR it might mean being pinned down by enemy fire in a military engagement with a need to fight one's way out. The picture that I get in all of it, is of a second working a duel. His job was to see that the guns were loaded and that the rules were observed, but he also was required to continue the fight if necessary when the person he supported went down. Truly a deadly scenario. Just some thoughts for what it's worth.
Like "riding gunshot" in the old west days? It does sound good. It means someone cares enough about you to keep on eye out on your behalf. I've had folks do that and found out about it later. Fight on my behalf with those about whom I knew nothing who were very insulting about me. I guess that's what friends do. Look out for one another. Thank you for your reply DA and Happy Saturday! :)
Benedict Arnold 