lugsail

/ˈlʌɡseɪl/
noun
  1. A four-sided sail attached to a yard that is slung at an angle to the mast, common on small boats.
    • They replaced the torn lugsail with a new one before the race.
    • A lugsail is easy to handle and works well for small sailing craft.
    • The fisherman raised the lugsail to catch the morning breeze.