Definitions:

1. In nautical terms, it refers to the position of a sail when the wind catches it from the opposite side, causing the sail to fill in the wrong direction. adverb
   • the ship came up into the wind with all yards aback
2. To be very shocked or surprised. adverb
   • I was somewhat taken aback by her honesty.

Synonyms:

• all of a sudden
• confused
• suddenly
• surprised
• unexpectedly