This idiom or expression means:

Watching someone’s back for support.

A. Hey Joe, I don’t know if I should do this?!
B. Go ahead, I have your back. I’ll help you if necessary.

This idiom or expression means:

A problem that won’t go away.

My house is definitely a monkey on my back now. I can’t afford to keep it, and I can’t sell it either!

This idiom or expression has to do with:

Hitting with the back of the hand.
Returning a shot with the back of the hand.

She threw him the back of her hand.