from
The Collaborative International Dictionary of English v.0.48
Aback \A*back"\ ([.a]*b[a^]k"), adv. [Pref. a- + back; AS. on
b[ae]c at, on, or toward the back. See {Back}.]
1. Toward the back or rear; backward. "Therewith aback she
started." --Chaucer.
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2. Behind; in the rear. --Knolles.
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3. (Naut.) Backward against the mast; -- said of the sails
when pressed by the wind. --Totten.
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{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.
[1913 Webster]