All the best phrases have nautical origins. So it is with taken aback, meaning ‘surprised or startled by a sudden turn of events.’ Aback means in a backward direction. Like ‘adown’ and ‘around’, it was originally two words, but these became merged into one in the 15th century.
Taken aback, then, is an allusion to something that is startling enough to make somebody jump back in surprise. The first to be ‘taken aback’ were not people, but ships. The sails of a ship are said to be ‘aback’ when the wind blows them flat against the masts and spars that support them. In this way, if the wind were to turn suddenly so that a sailing ship was facing unexpectedly into the wind, the ship was said to be ‘taken aback’.
The figurative use of the phrase, meaning surprised rather than physically pushed back, came in the 19th century. It was used by the great Charles Dickens, amongst other people, who wrote in his American Notes of 1842: “I don’t think I was ever so taken aback in all my life.”