Go behind my back
背着我做某事

I'm a partner in this firm, and you went behind my back to Kim?
我是你的合伙人，可你却背着我跟Kim沟通？

Go over your boss's head
越级

You went over your boss's head? He's going to kill you when he finds out.
你越过你老板（跟别人沟通）？他知道了可饶不了你。

Talk to your boss's superiors
和你老板的上级沟通

Talk to your boss's superiors without him/her knowing.
在你老板不知情的情况下和他的上级沟通。

Talk behind my back
在背后议论我

He talks behind my back, but is nice to my face.
他在背后议论我，却在我面前对我很好。

Behind closed doors
秘密的；暗中

The meeting was behind closed doors so I don't know what they talked about.
这个会议是私密的，我也不知道他们说了什么。

Between you and me
你知我知；就咱俩知道

This is just between you and me. Nobody else knows about it.
这个事是你我之间的，别人不知道。

Hush-hush
秘密的

Let's keep this hush-hush okay. If the word gets out, we're done.
咱们悄悄的ok，如果走露风声咱就完了。

Down low
低调

We might be getting a raise, but keep it on the d.l. (down low) because I'm not 100 percent sure.
我可能要加薪，不过低调点，因为这事也不百分百确定。

Quiet
悄悄的

Please keep this on the q.t. (quiet) until it's official.
在这个确定前咱悄悄的啊~