其实这是方言，生活习语，略带粗口，最早使用的文字是叫“牛屄”， 牛逼，俗写牛B或牛逼是一个不雅的汉语口语词汇，之所以用B来代替，可能使用的人心理上感觉要好点了。至于B的意思就没什么可以解释了！

Funny question. It's a dirty word as well as a cool word in slang. It's often used by men, seldom for ladies. It means someone is so awesome and cool after doing sth amazing or out of others expectation. Such as: The football player kicked 3 goals in first half of the match, 太牛B了！ （to be polite, you could eliminate 'B', and just say: 太牛了！or 太牛X (牛叉)了！ Such as: Tom played a practical jokes with the English teacher on class, he was so NB! It's as popular as SB=傻B(sha)=傻逼 but I suggest you never use it.

B是一个副词，形容很厉害，比如傻B就是“傻得很厉害”，牛B就是“牛得很厉害”，装B就是“装得很厉害”。

This is really funny! OK B is 屄, you can copy and paste to look it up at www.iciba.com But people usually don't type the right character cuz it's rude and dirty, so they use 逼 or B It means you are a badass, or you are very good at something. For example: "I got my PhD in 2 years." "牛B啊！＂ ---- "Do you know she can dunk like a man?" "真牛B!＂ ---- You can emmit B when you say it. "He stayed up continuously for 3 days to play DOTA and didn't die." "牛啊！"

It is Beijing accent word earliest.At first,it is just a dirty word,the pronunciation of B is as same as屄 and 逼(bī) which is genital organ of the female animals.Now,牛逼 means powerful,strong,exellent and well-down.We even have invented a Chinglish word “niubility”，haha.It is just a slang.