Source : Webster's Revised Unabridged Dictionary (1913)

Lugsail \Lug"sail`\, n. (Naut.)
   A square sail bent upon a yard that hangs obliquely to the
   mast and is raised or lowered with the sail. --Totten.

Source : WordNet®

lugsail
     n : a sail with four corners that is hoisted from a yard that is
         oblique to the mast [syn: {lug}]