The overview definition of "yeah" in Merriam-Webster's free online dictionary is obviously wrong ("—used in speech to show that you are surprised by or disagree with what someone has said"). However, this wrong definition was certainly not consciously entered for this word. Instead, presumably an editor misplaced the overview definition of some other entry there. So, the question is, can someone guess, correctly, what entry the spurious overview definition came from?
The entry the spurious overview definition came from definitely is Yeah.
yeah: 2 -- used in speech to show that you are surprised by or disagree with what someone has said.
"I'm from Maine." "Oh Yeah? [=really?] I didn't know that."
"I'm a better runner than you." "Oh, Yeah? You think you can beat me?"
"You're a lousy golfer." "Oh Yeah? I'd like to see you do better."