What does aback mean?
The definition of aback
A*back”, adv.
Etym: [Pref. a- + back; AS. on bæc at, on, or toward the back. See Back.]

1 Toward the back or rear; backward. “Therewith aback she started.” Chaucer.
2 Behind; in the rear. Knolles.
3 (Naut.)
Backward against the mast;-said of the sails when pressed by the wind. Totten. To be taken aback. (a) To be driven backward against the mast; — said of the sails, also of the ship when the sails are thus driven. (b) To be suddenly checked, baffled, or discomfited. Dickens.

Ab”ack, n.
An abacus. [Obs.] B. Jonson.