Wide-mouthed, like the mouth of a sparrow: it is said of such persons, that they do not hold their mouths by lease, but have it from year to year; i.e. from ear to ear. One whose mouth cannot be enlarged without removing their ears, and who when they yawn have their heads half off.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sparrow * Spatch CockNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes