Surprised or startled by a sudden turn of events.
'Aback' means in a backward direction - toward the rear. It is a word that has fallen almost into disuse, apart from in the phrase 'taken aback'. Originally 'aback' was two words: 'a' and 'back', but these became merged into a single word in the 15th century. The word 'around' and the now archaic 'adown' were formed in the same way.
'Taken aback' is an allusion to something that is startling enough to make us jump back in surprise. The first to be 'taken aback' were not people though 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. A use of this was recorded in the London Gazette in 1697:
"I braced my main topsails aback."
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'. An early example of that in print comes from an author called Eeles in the Philosophical Transactions of the Royal Society, 1754:
"If they luff up, they will be taken aback, and run the hazard of being dismasted."
Note: 'to luff' is to bring the head of a ship nearer to the wind.
The figurative use of the phrase, meaning surprised rather than physically pushed back, came in the 19th century. It appeared in The Times in March 1831:
"Whigs, Tories, and Radicals, were all taken aback with astonishment, that the Ministers had not come forward with some moderate plan of reform."
Charles Dickens also used it in his American Notes in 1842:
"I don't think I was ever so taken aback in all my life."
See other Nautical Phrases.