I'm reading "The Professor and t;he Madman" by Simon Winchester. On p. 7:
"Lambeth Marsh was also as it happened, just beyond the
legal jurisdiction of both the Cities of London and Westminster.
It belonged administratively - at least until 1888 - to the
County of Surrey - meaning that the relatively strict laws that
applied to the capital's citizens did not apply to anyone who
ventured, via one of the new bridges,like Waterloo, Blackfriars,
Westminster, of Hungerford, into the wen of Lambeth.

None of the usual meanings of wen seem to fit this passage.
If it were meant in the ususal sense of blemish, it seems too weak.
A wen in the sense of a wart or sebaceous cyst does not suggest
an ugly blemish. Lambeth is described as "lubricious" with illicit sex and
veneral disease rampant pollution from primitive industrial enterprises,
including tanneries dependent on collection of dog feces as tanning agent.
It sounds more like a carbuncle than a wen. Unless Winchester has some special
definition of "wen" more loathesome than any in my dictionary.