Dac that - It's someone who's so dumb they might as well be dead. Like

It's someone who's so dumb they might as well be dead. Like they walked into a burning building and said 'this is the best day of my life.'

'Dac that' when your friend walked into a burning building and didn't know what was happening.

'Dac that' when you tried to explain why you're alive and they just stared at you.

'Dac that' when your mom said 'why did you do that?' and you had no idea.