在背后说人闲话

behind sb.'s back
in the dark
in secret
on the sly
surreptitiously

wag one's tongue 多言; 饶舌

在背后: behind the others. behind 本来就是在...后面,因此背后就不再用单词来表达了.

如是所知的特定一群人,如同事中, 用the others
不指特定,的泛指, others

Wag one's tongue behind others.

背后：behind someone's back

gossip at other's back 或者
gossip behind others

talk behind somebody
英文就是这样说的。

gossip behind someone