The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
One that has a significant advantage: “a champion of the overdog who provides tax breaks for the rich while cutting social services for the poor” (Leon Daniel).
Origin: over + (under)dog.