1. protect yourself or somebody else from threat or danger
you better watch your back
When you 'watch your back' or 'watch someone else's back' it means to protect you or someone else from unexpected danger or from other people around you. This phrase is usually used as a warning not to trust others. If you promise to 'watch someone's back', it means you intend to defend them or stand up for them if needed. This is a casual phrase used more often in social contexts. Similar meaning expressions include 'watch out', 'take somebody under your wing' or 'beware'.