aback

• POS: Adverb
• HYPHEN: a=back

CIDE DICTIONARY

aback, adv. [Pref. a- + back; AS. on bæc at, on, or toward the back. See Back.].

•  Toward the back or rear; backward.  Chaucer.  [1913 Webster]
•  Behind; in the rear.  Knolles.  [1913 Webster]
•  Backward against the mast; -- said of the sails when pressed by the wind.  Totten.  [1913 Webster]

To be taken aback. (a) To be driven backward against the mast; -- said of the sails, also of the ship when the sails are thus driven. (b) To be suddenly checked, baffled, or discomfited. Dickens.

aback, n. 

 An abacus.  B. Jonson.  [1913 Webster]

OXFORD DICTIONARY

aback, adv.
1 archaic backwards, behind.
2 Naut. (of a sail) pressed against the mast by a head wind.

Idiom

take aback
1 surprise, disconcert (your request took me aback; I was greatly taken aback by the news).
2 (as taken aback) (of a ship) with the sails pressed against the mast by a head wind.

Etymology

OE on b{aelig}c (as A(2), BACK)

ROGET THESAURUS

aback

Rear

N rear, back, posteriority, rear rank, rear guard, background, hinterland, occiput, nape, chine, heels, tail, rump, croup, buttock, posteriors, backside scut, breech, dorsum, loin, dorsal region, lumbar region, hind quarters, aitchbone, natch, natch bone, stern, poop, afterpart, heelpiece, crupper, wake, train, reverse, other side of the shield, back, rear, hind, hinder, hindmost, hindermost, postern, posterior, dorsal, after, caudal, lumbar, mizzen, tergal, behind, in the rear, in the background, behind one's back, at the heels of, at the tail of, at the back of, back to back, after, aft, abaft, astern, sternmost, aback, rearward, ogni medaglia ha il suo rovescio, the other side of the coin.