sobersides
/ˈsoʊbərˌsaɪdz/
noun
- A serious, solemn, or humorless person.
- My uncle is such a sobersides that he never laughs at any joke.
- The teacher was a bit of a sobersides, always frowning at the students' antics.
- Even at the party, she remained a sobersides, sitting quietly in the corner.