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!
At least among the folks in my circles, in addition to the many fine answers already given, it refers to an irl action that is completely idiotic/unhinged that you’re doing for laughs (either your own or those around you).