Figuratively, an unanswerable objection: also a machine formerly used in Holland by robbers; it was of iron, shaped like a pear; this they forced into the mouths of persons from whom they intended to extort money; and on turning a key, certain interior springs thrust forth a number of points, in all directions, which so enlarged it, that it could not be taken out of the mouth: and the iron, being case-hardened, could not be filed: the only methods of getting rid of it, were either by cutting the mouth, or advertizing a reward for the key. These pears were also called pears of agony.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Choak * Choaking PyeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes