what is facetiousness?

urban dictionary for facetious:

Being facetious is when you say something you think is funny, but NOBODY laughs. So then you have to say “Oh, uh…I was being facetious”. And everyone’s like “Ooooh Kayyy… (What a jackass.)”

this is the closest thing that i feel adequately describes the word. found while having a discussion about what exactly it means to be facetious. it’s something i’ve never been able to pick up on and have been told i don’t do myself. sarcasm misses me too sometimes, but it’s much less common. i get sarcasm though, and am capable of being sarcastic. this one misses me, but i don’t know why.

update: morningstar has lit the way with some excellent knowledge. the most important of which that most people use both words incorrectly. sarcasm is meant to convey contempt and is more aggressive, while facetiousness is intended to be more neutral.

internet, can you help me understand? how would you describe this word/action?