On whose authority? According to whom? Used to express disagreement with or defiance against something that someone has just said. A: "There's no way a pipsqueak like you can win this race!" B: "Oh yeah? Says who?" A: "You can't take those files to the police!" B: "Oh, really? Says who—you?"
See also: Say
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.