Definition from Wiktionary, the free dictionary
(nautical) A square-rigged ship is taken aback when the wind blows on the front of the sails.
- (idiomatic, transitive) To surprise or shock; to discomfit.
- I was rather taken aback by his angry reply.
- The bad news took us aback.
- (nautical, usually passive) Of a ship: to catch it with the sails aback suddenly.
Usage notes 
- Most commonly found in the passive voice.
- Unlike most phrasal verbs, take aback in the active voice requires its object to immediately follow the verb. *The bad news took aback us is ungrammatical in contemporary English.
to surprise or shock