Have my back?

I noticed it when the king and his brother would refer to it. I think this is a modern saying in that context.

I don't know everything. Neither does anyone else

I can easily see that as an old saying. Back then, they fought with swords in battle. So I can see someone telling someone else "I'll have your back", meaning that they probably fight back to back, or he'll watch for people coming up behind.

When guns became popular, in particular, especially western shows it became "I've got your 6" I believe to mean 6 shooter. Then for a while it was used in reference to locations based on a clock face, not unlike we teach our teens to drive,,"hands at 10 and 2"...and in police cover "cover my 6" to refer to meaning anyone behind them. Then as police or military guns and the Sig Sauer gun became popular, the phrase became "cover my Sig" or "I got your Sig" to mean protectiing the partner from being by aiming their gun in the direction of where the suspect was so the other could make a move in position. But as to just how far back that phrase started (or if it was even just a clever TV or movie writer started it in show and it became popular that way!)