If someone is being facetious they’re being playful with an edge. A knock-knock joke isn’t facetious, but if you call it the most advanced form of comedy, you’re probably being facetious.
The word facetious comes from the French facétie for “joke,” and has come to describe a joke with a little drop of sarcasm. It used to imply “funny and witty,” as in, “Oh what a facetious chap!” But now it has taken on a darker tone, like a joke that’s not quite appropriate. Whether appreciated or not, facetious things are not for real.