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 spread roumers about somebody. 要是书面语言的话就会使用speak ill of sb behind his/her back
1.Do not talk against others behind their backs. 不要在背后说别人的坏话。 2.Please don't speak ill of me when I'm gone. 请不要在背后说我的坏话 3.She always speaks ill of men behind their backs. 她总是在背后说人家的坏话。 4.I won't hear anything said against them behind their backs. 我不能让谁背后说他们的坏话。 5.She talks about you behind your back all the time. 她总在背后说你的坏话。 6.We should not speak ill of others behind their backs. 我们不应该在背后说别人的坏话。 7.We ought not to speak evil of others behind their backs. speak ill of sb. behind one's back 我们不应该背后说别人的坏话。 8.We should not speak evil of a man in his absence. 我们不应该在背后说别人的坏话。 9.Such men as praise you to your face is to speak ill of you behind your back. 这种人当面表扬你而在背后说你的坏话。 10.To speak spitefully or slanderously about(another). 背后说…的坏话恶意地或诋毁地谈论（另一个人）