Dictionary.com
definitions

winter flounder

noun
  1. See under lemon sole.

Origin of winter flounder

An Americanism dating back to 1805–15

lemon sole

noun
  1. any of various popular food flatfishes, as Parophrys vetulus of the Pacific (English sole) and Pseudopleuronectes americanus of the Atlantic (winter flounder or blackback flounder).

Origin of lemon sole

1875–80; probably alteration, by folk etymology, of French limande dab (fish), Old French limande, lime, of uncertain origin
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for winter flounder

lemon sole

noun
  1. a European flatfish, Microstomus kitt, with a variegated brown body: highly valued as a food fish: family Pleuronectidae
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012