Definition of watersail in English:

watersail

noun

  • 1(Originally) †a spritsail (obsolete); (later) a small sail set beneath a lower studdingsail, or under the boom of various other sails, often reaching nearly to the water, and spread in calm weather.

  • 2A sail lowered into the water to act as a sea anchor.

Origin

Late Middle English; earliest use found in Naval Account.