Facetious

Facetious means "not meant to be taken seriously" or sometimes it can also used in a context to be super sarcastic.

E.G

A: I will give you 100 dollars if you do a front flip

B: B does front flip and demands 100 dollars from A

A: No, I cannot give you, I was being facetious when I offered a bet of 100 dollars

Here A did not really intend to give 100 dollars to B and hence it was being facetious.