Definition of sand flounder in US English:

sand flounder

noun

Regional
  • Any of various kinds of flatfish associated with sandy seabeds; now especially (US) the windowpane flounder, Scophthalmus aquosus, (NZ) the New Zealand flounder, Rhombosolea plebeia of the southwest Pacific (family Pleuronectidae), and members of the family Paralichthyidae.

Origin

Mid 19th century; earliest use found in Transactions of the Royal Society of Edinburgh.