You are going to watch out and be a second set of eyes for someone. You will look out for their best interests and let them know if they have missed something. You will stick up for them.
Dbyrn: Hey, Frappe, I saw some kids messing around near your car, looked like they might be think about breaking in.
Frappe: Dude, seriously?
Dbryn: Yeah, I took some pix of them with my cell phone and they saw me and split really quick.
Frappe: Thanks Dude!
Dbryn: Don't worry, you know I've got your back.
When you take the rap for someone else's actions, normally unwittingly and without consenting, this is a useful phrase.
Joe: Hey, what's going on Jack? Why is there a body in my room?
Jack: If anyone asks, you killed him, I tried to stop you, and there are no other witnesses. Now I must be going. Bye! *drives away too fast*
Joe: *shouting after Jack* You can't do that man! That's out of order! Don't you get it? You've dug your grave and thrown me in it! Now it's my grave! Man, you gotta come back!
"to lose one's mind" is an epigram meaning the subject whose mind was lost is thinking and/or behaving irrationally or inappropriately
With multiple daily conflicts with you, you couldn't give me a break one time, but the one time the tables are turned, you expect me to give you a break? You've lost your mind.