Originally the past tense of "to shit," but shat has evolved into something more. Shat doesn't share the harsh connotations of "shit," nor does it share it's seriousness.
Shat can be used in multiple fashions, such as "shatty," "shatting," "shatness," and even "shatmaster." It's uses are ambiguous, so even more exotic fashions of use are still evolving.
Shat nub, that shatty movie was, by far, the shattiest shat I've ever seen. It's shatness equaled 1000 Jack Thompson's in a single room. I fucking hate that shatting shatmaster.