When someone brings up a topic when talking but immediately drops it and refuses to switch back to the dropped topic, leaving the other hanging in the middle of a conversation. This annoying thing is called "Conversational Blue Balls". 

当某个人谈话间提到某个话题而忽然又转换话题避而不谈刚才的话题，把对方的胃口吊在那里。这种讨厌的事就是“对话蓝球”。

Here is an example of Conversational Blue Balls:

"Goodness, did you hear about what happened?" 

"What happened?" 

"Anyways, I'm gonna go." 

"Wait, what happened?" 

"Oh, don't worry about it."

看一个“对话蓝球”的例子：

“老天啊，你听说发生的事了吗？”

“发生了什么事？”

“没什么，我得走了。”

“等等，发生什么了？”

“哦，别管那个了。”

Dude, Jane TOTALLY gave me Conversational Blueballs yesterday. We were on AIM and she asked if I had heard about the party and she never answered when I asked about it.

老兄，简昨天真是给了我一个“对话篮球”。我们在AIM上聊天时她问我有没有听说过关于派对的事，我说没有，然后我问她派对的事她再也不回答我了。