What does aback mean?

Answer 1

It means "toward the back" just as "aside" means "toward the side".

The expression "taken aback" comes from sailing. In order for the sailboat to go forward, the wind must fill the back of the sail and push it forward. But if the sail is set at the wrong angle, the wind will fill the front of the sail instead, bringing it to a sudden stop and even pushing it backwards. Although this was sometimes used with large sailing ships as a kind of brake, generally it was not a desirable end. The reaction of someone who is stopped short by surprise was thought by sailors to be like a ship that was taken aback, so it was used metaphorically. Nowadays people mostly know only the metaphorical sense.

As an obsolete term, an aback refers to an abacus.