Etymology
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aback (adv.)

c. 1200, "toward the rear," a contraction of Old English on bæc "backward, behind, at or on the back;" see see a- (1) + back (n.). Now surviving mainly in taken aback, which originally was a nautical expression in reference to a vessel's square sails when a sudden change of wind flattens them back against the masts and stops the forward motion (1754). The figurative sense from this, "suddenly or unexpectedly checked or disappointed," is by 1792.

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Definitions of aback

aback (adv.)
having the wind against the forward side of the sails;
the ship came up into the wind with all yards aback
aback (adv.)
by surprise;
taken aback by the caustic remarks
From wordnet.princeton.edu