Literally 'to blow up an ox.' Origin story: Yellow River raft-makers had to inflate ox-hide rafts by blowing into them — an impossible-sounding feat, hence 'blowing an ox' = talking nonsense.
Wǒ bùshì chuīniú, zhè jiàn shì wǒzhēn de néng zuòdào.
I'm not bragging — I really can do this.
Tips
history
The full version is 吹牛皮 (chuī niúpí, 'blow up an ox hide'). On the Yellow River, people made rafts from inflated animal skins. Blowing up a whole ox hide by mouth was considered impossible — so someone claiming they could was obviously a braggart.
usage
Very colloquial and common in daily speech. 别吹牛了 (Don't brag!) is something you'll hear constantly. The related noun 牛皮 can mean 'bragging' in slang: 吹牛皮 = 吹牛.
