What is the meaning of Aback?

Towards the back or rear; backwards.

In the rear; a distance behind.

By surprise; startled; dumbfounded. (see usage)

Backward against the mast; said of the sails when pressed by the wind from the "wrong" (forward) side, or of a ship when its sails are set that way.

An inscribed stone square.

behind, in the rear

away, aloof, off

since, ago

Source: wiktionary.org