Having someone's back is an idiomatic expression meaning that you're watching out for that person, basically taking care and protecting that person from harm.

Backing someone up is also an idiomatic expression meaning that you will support that person in word and deed.
by ... Zjdbckdnznsjd September 5, 2019
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Look out or backs you up, supports you in whatever choices or decisions you make, protects you from harm in the world as well as protecting you from harmful people. Sticks up for you when or doesnt allow others to or are talk/talking about you
Just know I HAVE YOUR BACK no matter what you decide.

Dont worry I'll HAVE YOUR BACK if anything happens
by JujubetTheMinion November 2, 2019
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when you have a friend that would do you a solid or mutual respect
tom said "yo man can you help me with my relationship issues? "

And toms best friend, Bill said " ya i have your back"
by haileyc_111 August 3, 2014
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