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.
ab urbe condita
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