Not serious (7)
(Other definitions for trivial that I've seen before include "Banal" , "Mundane" , "Petty, piffling" , "Of little value" , "Worthless" .)
(Other definitions for flippant that I've seen before include "Light-hearted" , "showing disrespectful attitude" , "Irreverent, saucy" , "Cheeky" , "light" .)