(Variations: sez or sezz may replace says) An exclamation of defiance, disbelief, mere pugnacity, etc; izzatso: I'm in the wrong seat? Says who?(1926+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with says who
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