Green’s Dictionary of Slang

fen n.

[SE fen, a marshy bog; the image is of the ‘dirtiness’ of the prostitute or the receiver]
(UK Und.)

1. a prostitute.

implied in fag the fen under fag v.1
[UK]New Canting Dict. n.p.: fen a Strumpet, or Bawd, a common Prostitute.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[US]Matsell Vocabulum.

2. a madame, a procuress.

[UK]Partridge DU 235/1: ‘1725; † by 1880’.

3. a receiver of stolen goods.

[UK]Partridge DSUE (8th edn) 385/2: late C17–18.