not serious about a serious subject, in an attempt to be funny or to appear clever(Cambridge)
trying to appear funny and clever at a time when other people do not think it is appropriate, and when it would be better to be serious(Oxford)
خوش مزاج
He's just being facetious(Cambridge)
Stop being facetious; this is serious.(Oxford)
We could spend all our income on health care," she said facetiously.(Cambridge)
facetious remarks
facetious comments
Tongue-in-cheek
Jokey
Serious
Significant
Facetiously
Facetiousness