knock somebody down with a feather 为什么是使某人惊奇的意思?

网友回答

这是用在非正式口语中的习惯用语,用来作感叹语.
参考如下来自牛津高阶英汉双解词典的解释：
you could have knocked me down with a feather (infml 口) (used esp as an exclamation 尤用作感叹语) I was amazed 我很惊奇.