吹牛皮

吹牛皮:

Literally means:

blow the bull hide; brag

Actually means:

Bull hide was once used to make rafts on the river. The raft had to be blown up before sailing on the water, and that is to blow the bull hide raft.

Later, to blow the bull hide figuratively means boasting or talking big of sth. Derogatory.

牛皮是从前渡河用的牛皮筏(小船)。皮筏要靠人用嘴巴给它吹满了气才能浮在水上，这就是吹牛皮筏。

后来用“吹牛皮”比喻说大话，即把事实夸大了。有说大话（过头的话）不费力的意思。具有贬义色彩。