simpleton definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a person who is stupid or easily deceived; fool
Origin: ; from simple, after proper names ending in -ton
simpleton definition by American Heritage Dictionary
A person who is felt to be deficient in judgment, good sense, or intelligence; a fool.
Origin: simple + -ton (as in surnames such as Chesterton, Singleton).