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Wiktionary
take aback

vb. 1 (context idiomatic transitive English) To surprise or shock; to discomfit. 2 (context nautical usually passive English) Of a ship: to catch it with the sails aback suddenly.

WordNet
take aback

v. surprise greatly; knock someone's socks off; "I was floored when I heard that I was promoted" [syn: shock, stun, floor, ball over, blow out of the water]