say something behind someone's back

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.
See also: back, behind, say
Farlex Dictionary of Idioms.

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.
See also: back, behind, say, something
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • say (something) behind (one's) back
  • dish the dirt
  • dish on
  • dish on (someone or something)
  • a dog that'll bring a bone will carry a bone
  • dish the dirt on (someone or something)
  • grapevine
  • hear (something) through the grapevine
  • hear something on the grapevine
  • hear something through the grapevine