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1. Shit Yeah
A much more intense way of the popular 'Hell Yeah' phrase.
'Dude, that was awesome!'
'Shit yeah!'
2. shit yeah
the best possible way affirmative response
"Do you want to come over for chicken and dumplings?"

"Shit yeah, I do."
3. Shit yeah
1) a very enthusiastic way of agreeing.
2) exclaiming that you have forgotten something
person 1 "dude, are you pumped?"
person 2 "SHIT YEAH!"

or

person 1 "we need *extra item here*"
person 2 "shit yeah ! i'll get that!"

shityeah