Thousand Island dressingn. A salad dressing typically made with mayonnaise, chili sauce, ketchup, and finely chopped vegetables such as pickles, bell peppers, green olives, and onions.[After the Thousand Islands (perhaps in reference to the many small pieces of vegetables in the dressing).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Thousand Island dressing n (Cookery) a salad dressing made from mayonnaise with ketchup, chopped gherkins, etc [probably from the Thousand Islands]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Thou?sand Is?land dress?ing n. mayonnaise seasoned with chopped pickles, pimientos, hard-boiled eggs, etc. [1915?20] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.