slang The mouth of someone who has dental braces. Also used as a mocking nickname. I never used to smile for photos when I was in middle school because I hated the way my tin grin looked. Nice smile, tin grin! Don't get too close to any magnets, or you'll get stuck!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. a smile with a mouth having braces. I’ll be glad when I get rid of this tin grin.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.