You shouldn't gossip about people behind their back(s).你不应该背地里议论他人。

If you have something to say, why not say it to my face instead of whispering it behind my back?!如果你有话要说，干吗要背地里议论而不当着我的面说？

She went behind his back and spoke directly to his supervisor.她背着他直接向他的上司报告。