Emily Dickinson Lexicon

A-BACK', adv. [a and back, Sax. on bæc; at, on or toward the back. See Back.]

Toward the back; on the back part; backward. In seamen's language it signifies the situation of the sails, when pressed back against the mast by the wind. Taken aback, is when the sails are carried back suddenly by the wind. Laid aback, is when the sails are purposely placed in that situation to give the ship sternway. – Mariner's Dict.