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