From Longman Dictionary of Contemporary Englishbehind somebody’s backbehind somebody’s backSECRETif you do something behind someone’s back, you do it without them knowingI don’t like the idea of the two of them talking about me behind my back.I should have realized that he’d go behind my back (=do something without telling me). →back
Examples from the Corpus
behind somebody’s back• I'm not happy about you going behind my back like that. You should have told me.• People laughed at him behind his back.• He agrees with his boss to his face, but then criticizes him behind his back.• I'm sure the other girls are talking about me behind my back.• I thought you were my friend. Now I find you've been talking about me behind my back.• His name is Mr McLeod, but his studentscall him "BigNose'' behind his back.• Do you think people are talking about you behind your back?