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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. © 2015 Farlex, Inc, all rights reserved.
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 Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.