Can mean a variety of things, like a long string of VERY unfortunate or horrible events in a short time, but mainly used on internet forums to describe threads full of flaming, raging, trolling, etc all in one. Generally about religion vs atheism, ponies vs anti-pones, furries vs anti-furries, etc.
When a large flock of birds, flying overhead, all simultaneously release their bowels, spilling their shit all over you in a rain-like pattern.
Guy 1: Oh my god! Those bird just shit all over us!
Guy 2: Yes, it was a real shitstorm, I'll tell you!
Guy 1: Good thing I had my umbrella. The shit fell like rain.
Guy 2: Lucky you.
A kickass song by Strapping Young Lad. Very awesome to headbang to. A shitstorm is also precipitation of no other thing than shit. which would seriously suck for 2 reasons... cleanup, and the stench
person1: hey what's that smell??
person2: oh shit, the sky's gonna be shitting again! get the ponchos and gasmasks! We've got a shitstorm coming