Behind Someone's Back Meaning
(idiomatic) Without somebody's knowledge; secretly.
Example: The employees talked about their boss behind his back.
1908, W[illiam] B[lair] M[orton] Ferguson, Zollenstein, New York, N.Y.: D. Appleton & Company, OCLC 29686887 , chapter II:
I had never defrauded a man of a farthing, nor called him knave behind his back. But now the last rag that covered my nakedness had been torn from me. I was branded a blackleg, card-sharper, and murderer.