mock-serious
adjectiveUK /ˌmɒkˈsɪəriəs/
US /ˌmɑːkˈsɪriəs/
Pretending to be serious in a funny way.
He gave a mock-serious nod before telling the joke.
OTHER MEANINGS
2. Acting solemn to make fun of someone or something.
Her mock-serious tone made everyone laugh at the silly rule.
3. Looking earnest but not truly meaning it.
The teacher's mock-serious face hid a smile.
4. A joking imitation of a serious manner.
The play used mock-serious music for the clumsy hero.