sobersides

sobersides

A very stern, serious, solemn person, especially a man. The actor, usually known for his over-the-top comedy antics, here plays a sobersides whose internal demons threaten to swallow him whole. Don't mind him—he's just a humorless old sobersides.
Farlex Dictionary of Idioms.

sobersides

n. a very serious person; a grumpy old man. Old sobersides can’t manage to laugh at anything.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • swear on
  • swear on (someone or something)
  • gonzo
  • hams
  • swear on a stack of Bibles
  • swear on a stack of Bibles, to
  • funny man
  • swear up and down
  • swear blind
  • swear blind...