Who claims the truth of this statement, which I don't believe? For example, That horse of your will never win a race.—Says who? This slangy expression of disagreement or disbelief may also be put in declarative form, says you!, meaning “I disagree with what you just said.” For example, It was an accident, I didn't mean to break it.—Says you! You've always hated that vase. [First half of 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.