push the envelope字面意思是“推信封”，但实际意思跟信封没有关系，真实含义是挑战自我、挑战极限、挑战限制尺度、突破原有底线。

【例句】
If you want to make a name for yourself in the art world, then you can't be afraid of pushing the envelope.
你如果想成名，就不能怕挑战极限。

Well, he certainly likes to push the envelope, but he's also a loose cannon.
嗯，他肯定喜欢打破常规，但他也是一个我行我素的人。