What is the correct way to write 'niu bi', and how did it get its meaning in Chinese? 'niu bi' (牛逼? like the bees knees in English) is one of the more amusing expressions I learnt in my time in China. The literal meaning in English is quite colourful, so I won't describe it here, but I'm curious to know more about this term.

What is the correct way to write this word, and how did it originally get it's meaning in Chinese?
niu bi = f**king awesome in English.
And niu in slang means awesome.

The usage is pretty same as in English.

For example:

A：我能一连吃10个汉堡.
   I can eat 10 burgers in a row.
B：牛啊！
   Awesome!

A: 你好牛，这么难得题目你也会做！
   You are so awesome. You can even do such difficult questions!
B: 谢谢。
   Thanks.


Additionally, regarding to 牛X or 牛叉.
It is common in online forums/tweets/blogs, but it is rare in conversations.
Reason is same as people write LOL online very often but seldom say L-O-L in conversations.

If you want to be polite, say 牛 instead of 牛X or 牛叉