吹牛 (chuī niú)

The idiom "吹牛 (chuīniú)" combines "吹 (chuī), to blow" and "牛 (niú), cow", originally symbolizing exaggeration, like blowing something up to make it seem bigger. Over time, "吹牛 (chuīniú)" came to mean boasting or bragging, where someone exaggerates their abilities or achievements.

For example:

Tā zǒng shì xǐ huān chuī niú, shuō zìjǐ néng jiě jué suǒ yǒu wèn tí.
他总是喜欢吹牛，说自己能解决所有问题。
He always likes to boast, claiming he can solve all problems.

Source:eChineseLearning