“背后说别人坏话”用英语怎么说？

英语谚语：
Don't speak ill of others behind their backs.
不要在背后说人坏话。

speak ill of sb. behind one's back
e.g
Don't speak ill of him behind his back.别在背后说他坏话
We should not speak ill of others behind their backs`
我们不应该在背后说别人坏话

to bad-mouth (或 speak ill of) someone behind his/her back

speak ill of others behind their backs

speak evil of somebody in his/her absence

再简单点。背后诽谤backbiting

背后说别人坏话
blackmouth sb in behind

to talk behind somebody's back, 一般都是这样说，但是要是指散布谣言，那就会说 spreading roumers about somebody.
要是书面语言的话就会使用speaking ill of sb behind his/her back