Derives from the term 'shipwreck'.
The result of peeing and pooing at the same
time under precise circumstances that causes the poo to be almost
lost in the merky
water – baring an eery resemblance to a shipwreck.
John: "There you go, mate.
Sorry for taking so long..."
Lee: "It's no bother – WOAH!"
John: "Huh?"
Lee: "Why the blooming fuck is there a shrunken replica of the Pirates of the Carribean set in my
toilet?"
John: "Fuck, I thought I already flushed! Shitwrecks can be
hard to spot..."
Lee: "You better flush this thing into Davey Jones' locker right now!"