Aback

adv. [From a and back; A-S. on bæc, at, on, or toward the back. See BACK.]

1. Toward the back or rear; backward; back. “Therewith aback she started.” Chaucer.
2. (Naut.) Backward against the mast; — said of the sails when pressed by the wind. Totten.

Hence, taken aback, or all aback, denotes suddenly checked or disappointed; unexpectedly baffled.