Why is London called the Great Wen?

'Wen' in old English was considered similar to the word 'boil' - a festering heap of corruption.

London in the Eighteenth Century Georgian period was expanding rapidly, especially in the new Western neighborhoods. However, not everyone appreciated this spread of urbanization. The author Daniel Defoe (1661-1731) called London "the monstrous city". Josiah Tucker (1713-1799), an economist and political writer, wrote that London was "no better than a wen". Finally, the radical journalist and politician William Cobbett (1763-1835), himself a critic of industrialization, adapted the phrase. In 'Rural Rides' (1830), he wrote: "But, what is to be the fate of the great wen of all? The monster, called, by the silly coxcombs of the press, 'the metropolis of the empire?'"