The first time somebody said to me, "I've got your back," I have to admit, I didn't know what he was talking about!
My eyes were looking ahead of me at the person I was talking to while my mind was visualizing somebody standing behind me. NO, there was no threat from behind me. My mind went back to what I could see. I had an inkling what this phrase refered to but if I were being threatened by somebody, in front of me would be the place I'd want somebody to be standing if they were trying to protect me!
I understand this phrase assumes that a threat will come from behind, which isn't always the case. I can tell you. The stranger who was holding a very large knife to me in my own apartment was, one morning, standing quite in front of me, not behind me. All I had to do was to open my eyes.
Be that as it may, this phrase "I've got your back" has now taken off. I hear it on TV, on action programs and movies, in book titles. Huffington Post did a piece on this in 2014. but the post seems to be making exactly my point: That the people we are most afraid of, and where people often feel the greatest threat, is from those we know, those who are standing right in front of us! Maybe even those we live with!
(The people who I know, whom I most trust, would never say that, by the way.)
To keep this post short, let's just say that the phrase "I've got your back" means "I'll protect you."