literally, NIU BI=cow cunt(NIU DE BI, cow's cunt), so it's coarse language.
NIU BI originated from CHUI NIU BI (course, not used in public or in writing), from CHUI NIU PI 吹牛皮( to inflate a cowskin raft; not taboo word originally), from KOUQI HENDA( literally, mouth air-stream very strong, KOUQI--manner of speaking, KOUQI HENDA--speak in a bragging manner)