boatswain's chair

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A short board secured by ropes and used as a seat when working aloft or on the side of a ship.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical A short board or other seating, secured in a bridle of ropes, used to hoist a man aloft or over the ship’s side for painting and similar work.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a seat consisting of a board and a rope; used while working aloft or over the side of a ship