The literal meaning, “to blow up a cow,” comes from a folk origin story. In the past, people in Northwestern China would cross rivers on rafts made from inflated sheep or cowhides. Inflating a sheepskin was hard enough, but inflating an entire cowhide by mouth would be a superhuman, impossible feat. Therefore, if someone claimed they could `吹牛` (blow up a cow), it was the ultimate, unbelievable boast. The phrase stuck and is now the most common way to talk about bragging.
`吹牛` is a culturally rich term because it stands in direct opposition to the traditional Chinese value of `谦虚 (qiānxū)` - modesty and humility. In Chinese culture, openly boasting about one's accomplishments is often seen as a sign of immaturity, insecurity, and poor character. A truly capable person is expected to be humble and let their actions speak for themselves. This contrasts with Western, particularly American, culture, where a degree of “self-promotion” is often seen as necessary and even admirable. In a job interview or business pitch in the US, you are expected to highlight your strengths confidently. Doing the same thing in a Chinese context, if not done with extreme care, could easily be perceived as `吹牛`. Therefore, `吹牛` is more than just “bragging”; it's a violation of a social norm. Accusing someone of `吹牛` is a way to socially check them and remind them of the importance of humility. However, among close male friends, a bit of light-hearted `吹牛` over drinks can be a form of social bonding, much like “shooting the bull” or “talking trash” in a friendly way.
`吹牛` is an extremely common, informal term used in everyday life.