E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
An argument to which there is no answer. Robbers in Holland at one time made use of a piece of iron in the shape of a pear, which they forced into the mouth of their victim. On turning a key, a number of springs thrust forth points of iron in all directions, so that the instrument of torture could never be taken out except by means of the key.