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It's French for 'in agreement' but nobody cares because it's just a fancy word for 'okay' and that's not worth getting a PhD for.
'Dac that' when your mom says 'just go to sleep' and you're too tired to argue.
'Dac that' when your friend says 'we can do this later' and you know they'll never do it later.
'Dac that' when your teacher says 'no homework' but you're still gonna do it anyway.