In Australia it means that you're in a lot of trouble, with no foreseeable way out.
"Oh man, I can't believe I lost my paperwork; that stuff took me hours and it needs to be in by tomorrow!"
"Sounds like you're up shit creek without a paddle, mate."
It is a Portuguese expression, translated to "desenmerda-te!", that is used when you want to say to someone that they're stuck in the mud and they need to sort their stuff out.
Nao ha comboio hoje? Vens a pe! Desenmerda-te! Tens de ca vir ter - Is there no trains today? Come on foot! Unshit yourself. you've got to come here!