Britannica Dictionary definition of YEAH
informal
1
:
yes
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“Are you coming with us?” “Yeah, I'm coming.”
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Yeah, I agree with you.
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“That looks good.” “Yeah, I think so too.”
2
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used in speech to show that you are surprised by or disagree with what someone has said
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“I'm from Maine.” “Oh yeah? [=really?] I didn't know that.”
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“I'm a better runner than you.” “Oh, yeah? You think you can beat me?”
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“You're a lousy golfer.” “Oh yeah? I'd like to see you do better.”
3
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used in speech to express disbelief