A friend had a high-school classmate who spent every physics class staring at the ceiling, either asleep or completely indifferent. No matter how angry the teacher got the classmate never did the least bit of work, and his attention always remained fixed on the ceiling. When the semester was over and the test results came out, the classmate scored nearly 100%. The classmate was niubi.The explanation is probably good enough to convey the concept, and the concept is not at all obscene. If you want to understand why the term is, you can follow the link; however, given the sexual nature of the obscenity, I'll trust that you will all be delicate in the comments if you decide to discuss it.
One thing I find amusing is that the Chinese are apparently often too embarrassed by the term to use it in polite company, but they find that they can't do without the concept, so they just pronounce the first part of the word. That part of the word means "cow." So, someone does something cool and laid back, and you'd say, "He's so cow."
Speaking of Chinese obscenities, apparently the good folks in Taiwan weren't too happy about today's op-ed in the New York Times, "To Save Our Economy, Ditch Taiwan." The Taiwanese have their own special way of expressing themselves on these points.