The face, or rather the mouth: from the German word MUND, the mouth. Toute his muns; look at his face.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Mung * Munster PlumsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes