Mumbling a sparrow; a cruel sport frequently practised at wakes and fairs: for a small premium, a booby having his hands tied behind him, has the wing of a cock sparrow put into his mouth: with this hold, without any other assistance than the motion of his lips, he is to get the sparrow’s head into his mouth: on attempting to do it, the bird defends itself surprisingly, frequently pecking the mumbler till his lips are covered with blood, and he is obliged to desist: to prevent the bird from getting away, he is fastened by a string to a button of the booby’s coat.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sparking Blows * Sparrow-mouthedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes