Green’s Dictionary of Slang

(also rat-trap mouth) the mouth; thus rat-trap-mouthed adj.

1899		C. Rook Hooligan Nights 52: That [i.e. a wooden gag] beat all uvver ways of stopping rat-traps.
1908		A.N. Lyons Arthur’s 116: If you don’t keep that rat-trap shut, I’ll —.
1947		J. Maclaren-Ross Of Love And Hunger 116: Dour old boy. Rat-trap mouth, red bald head.
1965		G. Melly Owning Up (1974) 87: Most of the boarding houses were run by rat-trap-mouthed women married to Pols.
1979		J. Morrow Confessions of Proinsias O’Toole 88: ‘Shut yer puggin’ face!’ he reparteed grimly, fag wagging in the centre of his functional little rat-trap.