gibface, n. — Green’s Dictionary of Slang

gibface n.

[Hotten (1860) states ‘properly the lower lip of a horse’ but this is not in OED or EDD; poss. mistake for dial. gib, the upper lip of a fish; note also dial. gib, a hook, thus gib-nosed, hook-nosed]

an ugly person, esp. one with a heavy lower jaw.

[1533		J. Heywood Play of Love in Farmer (1905) 172: A woman that other so ugly were / That each kiss of her mouth called you to Gyb’s feast].
1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 147: GIB-FACE, properly the lower lip of a horse; ‘to hang one’s gib,’ to pout the lower lip, to be angry or sullen.
1873		Sl. Dict.
1891		A. Day Mysterious Beggar 230: He’s a reg’lar gib-face.