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1.
One who has metal in their mouth.
Commonly referred to as "braces"
"Dat bitch is a tin grin!
lolz"
by BUBBASPARKZZZ October 21, 2007
 
2.
a smile on a person with braces
I think he would get more compliments if he didn't have that tin grin.
by The Return of Light Joker January 18, 2010