我知道你在吹牛 英文怎么说?

I know you are bragging/bluffing.

仅供参考。

I know it is your bullshit/

I know you are boasting
I know you are giving a whopper

I know you are shooting the bull.
shoot the bull 就是说大话，吹牛的意思。。。

I know you're talking big

I do think you are telling me a story.