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!
You can also avoid pronuncing the "t" in shit; it's more classy.
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.
girl: I broke a nail again.
boy: Shitdemârde! (said with sympathy)
girl: I lost my job!
boy: Shitdemârde! (said with alarm)