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1.
This is a common sarcastic response to any exclamation of enthusiasm or joyous anticipation. It's intention is to present your general disinterest
or disliking for their suggestion.

Infact it could be said that the "yeah" actually means "no". This is evident by the highly sarcastic tone in which it should be spoken.
"I'm going to go and love up random people who once said hello to me now"

"Good luck with that, YEAHHHHH"
by morgangills February 26, 2005