Verb:
My piece of shit Mitsubishi just shit down in a Hooters parking lot. If it had done so next to a Taco Cabana, all of the parts would've been stolen and I wouldn't have had to pay to have it towed to a fucking junkyard!
When a person takes a dump, accidental or otherwise, the literal second their ass touches the toilet seat. Often associated with sudden/panic Diarrhoea, Irritable Bowel Syndrome, or just plain misfortune.
One:
"Thank fuck! Shitdown! A second later an' I woulda fudged my drawers fo' sho' "
Two:
A: "Why you all flustered dude?"
B: "Yo man just had a shitdown, coulda been some serious shardage"
A:"Thank the Lord"
Similar to a shutdown, which is when a large piece of industrial equipment is out of service for days/weeks for maintenance, a shitdown is an extended visit to the bathroom for defecation.
Guy 1: "Where's Steve? He said we were leaving at 8."
Guy 2: "Last night was taco night. I suspect he's on shitdown."
Hey man, are you ready for tomorrow's shitdown? We both know things are about to get tough... But I want you to know that, whatever happens, I'll make sure I'm the one stuffing shit down your throat. Never forget that, brother.