Said of sails pressed backward against the mast by the wind- hence, TAKEN ABACK, taken by surprise.
On the back; backwards, as used by sailors; by surprise; unexpectedly.
An abacus.
A long round piece of timber or iron tube, raised upright on the keel, through the decks, of a vessel to support the sails; any upright pole; the fruit of the oak, beech, etc., especially when used as food for swine. ...