Green’s Dictionary of Slang

metal mouth n.

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

[US] in Daily Beacon (TN) 10 Nov. 4: Painful kisses, baby food for dinner and nicknames like ‘metal mouth’ and ‘tinsel teeth.’ [HDAS].
OnLine Dict. of Playground Sl. [Internet] metal mouth! n. unkind name for someone wearing braces on their teeth.