From Shipbuilding Terms:
Jackyard Topsail: A triangular topsail set above the mainsail in a gaff rigged vessel.
From Nautical Terms (Global Marine Insurance):
A spar extended above the gaff used to provide greater spread to a topsail
An oversized gaff topsail using "clubs" to extend the mast and the gaff; also called "club topsail"
From Dixon Kemp's Dictionary (Manual of Yacht and Boat Sailing and Architecture): http://www2.friend.ly.net/~dadadata/kemp/index.html#to: