O que significa "to be taken aback"?

One of these days, my friend and I were discussing a very controversial topic when I said something that might have shocked him or taken him by surprise. After a few minutes, when our conversation wasn't even about that topic anymore, he brought up the fact again that he didn't expect me to have such opinion. Here's what he said: I must admit I was taken aback for a moment when you said that.

When you're taken aback, you're surprised and confused. Suppose you go out for dinner with your boyfriend one night and when you're at the restaurant he unexpectedly proposes to you. You are taken aback because you got surprised and confused at the same time. So much so your boyfriend didn't get any reaction for a few second until you said yes. If you pay close attention to the literal meaning of the expression, you'll see it's based on the idea of aback [backward], something you literally do when you're taken by surprise. See that reaction on the picture below: