say (something) behind (one's) back
To discuss, gossip, or remark upon someone when they aren't present or listening, especially when saying something negative. I can't believe you said those things about me behind my back—I thought we were friends! She so arrogant; if only she knew what people said behind her back.
say something behind someone's back
If someone says something about you behind your back, they say unpleasant things about you to other people. Friends were criticizing his wife behind his back. `Stop talking about me behind my back,' said Susannah, and burst into tears. Compare with go behind someone's back.