Facetious

adjective
joking or jesting, often inappropriately

A great word for a few reasons: First of all, it just sounds great. I like to throw it around in conversation, which is especially easy because I probably defend myself with "Oh, I'm just being facetious!" every few days. The best thing about facetious, though, is that it contains the five vowels in alphabetical order!

facetious

and sometimes:

facetiously