"You could have knocked me over with a feather" means that you were very surprised, even shocked by something. You were so shocked you couldn't even think. When a person is like that, you can easily push him over / push him off his feet to the ground. You don't even need to push hard with your hand. You can use a feather. It is an image, so no one really falls to the ground. ; )

它的意思大概是說，發話者( "I" )實在弱不禁風、又或是太脆弱了(身體上或知識、能力上)，導致受話者(you)可以光以一根羽毛就可把受話者輕易擊倒。 祇是說，這句話有點太謙了吧；但若反過來說，那就算是蠻囂張和挑釁的說法了吧。例如： "I could knock you over with a feather, bastard!!", said by A to B, who was arrested by attempting to pickpocket on A. 另外，在情態動詞後是用不定式(infinitive)而不是過去分詞(past participle)；所以應是 "could knock"。除非是過去時態，那麼才用上過去分詞，但須先加上 "have"，即 "could have knocked"。 祝　是日安好。/ Good day.