An interjection directing silence. Mum for that; I shall be silent as to that. As mute as Mumchance, who was hanged for saying nothing; a friendly reproach to any one who seems low-spirited and silent.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Mulligrubs * MumchanceNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes