YES U R RIGHT AND THIS IS NOT A FORMAL LANGUAGE ,JUST INFORMAL!!!

哈哈，我没听过这样的用法。我们会说“你好牛啊”或者“太牛了”，这些都是“很新潮”的词，如果说给长辈听，是很不合适的。但是你的理解基本正确。“牛”就是“很厉害”的意思:) I never heard that before. We sometimes say "ni hao niu a" or "tai niu le", but these are newly created by young people. It's not proper for you use these word when you talking to seniors. However, you are almost right. The word "niu" means "strong" "powerful" and also means "the ability to do something" "talent". When you find someone who is intelligent or good at doing something, or knows what you don't know, you can use this word to express your admire.

是的，而且这个词很多时候不是用在称赞