为什么hot air 是吹牛的意思,它有什么来源吗?

不是说话时人会喷出热空气吗?那么这个意思就是说,这个人说的话就是一团热空气,没其它的东西.指吹牛,不可信.比如"he's full of hot air".
参考：
"Hot air" refers to the breath one lets out as they speak.When we point out what they say is "a lot of hot air" – think about a hot-air balloon – we mean to say it's all empty talk.
If promises are "a lot of hot air",don't believe them.
Sometimes boastful remarks are referred to as "a lot of hot air",that means they are bragging to impress people – don't take them seriously.
Also common is the phrase "he's full of hot air".