"Shat bricks" is a popular and ubiquitous exclamation from Anonymous whenever he sees or realizes something extremely surprising. Typically used in reference to surprising/mind-blowing plot twists in various anime, videogames, comics, books, etc.
The meme started out in 'creepy threads' on /b/, when Anonymous posted several innocent-looking pictures that surreptitiously contained something very freaky if you looked at it hard enough (for instance, posting a picture of a forest where the branches came together to form the shape of a fetus) with the caption "when you see it, you'll shit bricks." The meme quickly took off and spread to the rest of 4chan, and since then, whenever anyone sees anything very surprising or unexpected on any board, they will "shit bricks." If they see something extremely surprising, they'll shit enough bricks to build a house.