Expression used when things go wrong. It is useful to make people get over a small incident or, on the opposite, it can be used to dramatize a situation. It's all in the intonation!

Notes:
You can also avoid pronuncing the "t" in shit; it's more classy.
Shi-demârde.
The sound or the "â" is longer, like the one in Mârdi grAs.
ps. Marde=shit in French so the expression depicts shit made of shit.