A noisy fellow. Mouth half cocked; one gaping and staring at every thing he sees. To make any one laugh on the wrong, or t’other side of his mouth; to make him cry or grieve.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Mousetrap * Mouth-1Nathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes