Green’s Dictionary of Slang

(US campus) a person wearing orthodontic braces.

1965		Delaware Co. Dly Times (Chester, PA) 5 Feb. 9/1: I had to live with the names ‘tinsel teeth’ and ‘iron mouth’ [...] it hurts.
1969		Cincinnati Enquirer (OH) 19 July 3/4: All the boys do is call me tinsel teeth and steel jaw.
1971		Current Sl. VI.
1981		Petosky News-Review (MI) 8 Jan. 4/3: She has been called ‘brace face,’ ‘metal mouth,’ tinsel tooth,’ and her favorite — ‘tin grin’.
1987		G.A. Fine With the Boys 169: Tinsel teeth, n. Boy who wears metal braces on his front teeth.
1990		Charlotte Obs. (NC) 11 Apr. 9D/3: Hey, tinsel teeth! There’s a mouthful of reasons to wear those wires.
2004		Atlanta Constit. (GA) 31 Jan. IE4/6: ‘Tinsel teeth’...‘metal mouth...nobody says that stuff any more.
2012		Tampa bay Times (St Petersburg, FL) 2 Jan. SF/1: [cartoon caption] ‘Ha Ha! Tinself teeth! Metal Mouth!’.