"So what does that have to do with the price of tea in China?" This is nearly precisely the same idiom in English, in tone, usage and circumstance. Hope that gives you what you need.

The speaker is talking about something, but the points or factors have nothing to do with each other or have no causal relationship , then the listener uses 这哪儿跟哪儿啊？ to mean that he can not get your point or what u r talking about is sort of ridiculous. EXAMPLE: 1. A ： 你吃过午饭了吧？ have u had your lunch? B： 是啊。 Yes , I have. A： 怪不得今天天气这么好。 no wonder today is a good weather day. B： 这哪儿跟哪儿啊.... TIPS: 1. we normally use 这哪儿跟哪儿啊 in the oral Chinese. 2. 这哪跟哪啊 can also be used. And when you pronounce it , you can add 儿 to make it sound more like northern accent. 3. 这什么跟什么啊 has the same meaning of it. This is more like ' what is what ' . 4. advanced idiom : 这简直就是关公战秦琼. This is 关公 fighting against 秦琼！ 关公 and 秦琼 were the famous warriors in ancient China, but they were in different dynasties. So it means what u r talking about is quite ridiculous.

It means what the you're talking about has nothing to do with the topic we discussed just now. Simply put, B has nothing to do with A. The example is from the following page: 甲： 如果伊拉克战争结束了(A)，瑞典就处在威胁之下(B)。 乙： 哪儿跟哪儿这是？ (What does B have to do with A?) http://en.wiktionary.org/哪儿跟哪儿 Hope it helps. :)

A：我看到你和那个女孩一起出去吃饭了，她是你女朋友吗？ B：哪儿跟哪儿呀，那是我的好朋友。

我觉得有两种意思吧：第一，一个人说话牛头不对马嘴，尽说些不相关的事的时候。你会说：“这哪儿跟哪儿啊”第二，别人说的事情你不赞同，比如上一个人举的例子，看到你跟一个女生在一起，别人问你“那是你女朋友么？”你会说：“这哪儿跟哪儿啊，只是朋友而已”