Definition from Wiktionary, the free dictionary
Etymology 1 
- From Middle English abak, from Old English on (“at, on, or toward”) + bæc (“back”).
- a- (“towards”) + back (“back”).
aback (not comparable)
- (archaic) Towards the back or rear; backwards. [First attested prior to 1150.]
- (archaic) In the rear; a distance behind. [First attested prior to 1150.]
- By surprise; startled; dumbfounded.
- (nautical) Backward against the mast; said of the sails when pressed by the wind from the "wrong" (forward) side, or of a ship when its sails are set that way. [First attested in the late 17th century.]
- By setting the foresail aback and the headsail in the middle one can bring a fore-and-aft rigged sailing boat practically to a halt even in heavy wind.
Usage notes 
- (by surprise): Preceded by a form of the word take.
in the rear
said of sails pressed backward
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Translations to be checked
See also 
Etymology 2 
aback (plural abacks)
abacus — see abacus
- 2003 , Brown, Lesley editor, The Shorter Oxford English Dictionary, edition 5th, Oxford, UK: Oxford University Press, ISBN 978-0-19-860575-7, page 2: