Green’s Dictionary of Slang

(US campus/UK juv.) a person with orthodontic braces.

1978		in Daily Beacon (TN) 10 Nov. 4: Painful kisses, baby food for dinner and nicknames like ‘metal mouth’ and ‘tinsel teeth.’ [HDAS].
1981		Petosky News-Review (MI) 8 Jan. 4/3: She has been called ‘brace face,’ ‘metal mouth,’ tinsel tooth,’ and her favorite — ‘tin grin’.
1998		Charlotte Obs. (NC) 10 Aug. 12: Share your happy or horrifying tinsel-tooth tales [...] ‘Yo! Metal Mouth,’ ‘ [...] ‘Hiya, Brace Face’.
2001		OnLine Dict. of Playground Sl. 🌐 metal mouth! n. unkind name for someone wearing braces on their teeth.
2012		Tampa bay Times (St Petersburg, FL) 2 Jan. SF/1: [cartoon caption] ‘Ha Ha! Tinself teeth! Metal Mouth!’.