It gets used online for things, and there are whole communities devoted to shitposting, but I can’t find a clear set of rules for something to count as a shitpost. I remember querying whether a post on a shitposting community was witty enough to be a shitpost rather than just a shitty post, but of course not all the responses to that were terribly helpful!
What about shart-posting?
Starts off OK but ends in disappointment?
Starts off with only a few words on the topic, but ends with writing a thesis on an unrelated diversion.