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.
"Thank fuck! Shitdown! A second later an' I woulda fudged my drawers fo' sho' "
A: "Why you all flustered dude?"
B: "Yo man just had a shitdown, coulda been some serious shardage"
A:"Thank the Lord"