A shitcoin is a coin that provides absolutely no value. Value is of course subjective, but you can often tell that a part has just been made to make a part.
Usually these are parts that have been put together without much thought as to how demand will be created for the part or what problems it will solve. Some are intentionally designed as jokes, others make the most of “rug-pull” technology, attracting unsuspecting noobs who will eventually get a rekt.
Of course, if you’re a bitcoiner, all other coins that aren’t bitcoins are shitcoins, which makes things much easier to understand!