First word in the Vest Pocket Dictionary: Aback’.
Naturally since this dictionary measures 10.50 by 6.00 cm, the actual definitions are quite… brief.
The book gives the definition for aback’ as “backward”.
Seriously, I warned you it was going to be brief, don’t be taken aback’.
Is it strange how I really enjoy such a bare bones definition of a word? When did we start to need such depth?
Either way, most of us English speakers would know that aback’ is an adverb, and as a reaction. You have been moved backward due to the force of something.