aback (adv.) Look up aback at Dictionary.com
c.1200, from Old English on bæc "at or on the back;" see back (n.). Now surviving mainly in taken aback, originally 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 of the ship (1754). The figurative sense is first recorded 1840.