Green’s Dictionary of Slang

bracket-mug n.

an ugly face.

1859		Matsell Vocabulum 14: bracket-mug. A very ugly face.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		Crowe Aus. Sl. Dict. 11: Bracket Mug, a very ugly face.