Recently I saw this cartoon and it really made me laugh. What a silly expression of a phrase that we use often for encouragement and strength for those we care about.
I remember (way back) in high school when I first heard this phrase and some tough guys were planning a fight. One of the guys said to his buddy, "I got your back."
Basically he was saying, "if you need me I'm right behind you"..."when things are going badly for you I will step in and be on your team"..."you're not alone"..."I'm with you in this".