From Longman Dictionary of Contemporary Englishwatch your backwatch your backinformal to be careful because other people may try to harm you → watch
Examples from the Corpus
watch your back• He may be prime minister for now, but he still needs to watch his back.• But Yeltsin will have to watch his back.• Drug traffickers and guerrillas mix with spies and mercenaries, all cautiously watching their backs.• Enjoy it, but watch your back.• She noticed that everyone seemed to have drifted a little closer; was talking a little less, watching their backs.• She watched his back for a moment.• Other agents are watching his back, he says.• Besides which, I needed an ally at the shop, some one to watch my back should things get hairy.• But watch your back when I get out, all right? 