greenback

noun4

1. •
   (US) Any bill that is legal tender in the US (originally printed with green and black ink) issued by the Federal Reserve.
2. •
   The United States dollar.
3. •
   (historical) A unit of American currency issued during the Civil War by the Treasury Department.
4. •
   A wave that has not yet begun to break.

   • When the surfer has passed the point where the waves are breaking, and is among the waves which are known as “greenbacks”, he may either dive through them or float over them, […] — 1965