Someone who is facetious is only joking: "I was being facetious when I told my mother I want Brussels sprouts with every meal, but she took me seriously!"
Facetious is a useful word to describe something that's humorous, or meant to be humorous. If a joke falls flat, then you can back off from it by saying you're only being facetious. There are limits to this use of the word: if you stage an elaborate prank on your friend, making him run out into the street in his underwear because he thinks his house is on fire, calling the joke facetious will probably earn you a punch in the face.
