Green’s Dictionary of Slang

maggyland n.

also mageland
[Madge Howlet n.]

the world of prostitution.

[UK]Mercurius Democritus 7 Apr. 4: Both their husbands were pure Virgins [...] till they arrived at the Gulph of Mageland.
[UK]Mercurius Fumigosus 6 5–12 July n.p.: News from the straights of Madgyland. [Ibid.] 10 2 Aug. 93: The Night-Mare being taken napping the other Night as she was going into Maggy-land.
[UK]Mercurius Democritus 24-31 May 25: Commencing many Strange Wonders Out of [...] The Antipodes, Maggy-land, Tenebris, Fary-land, Green-land, and other adjacent Countries.

In compounds

maggyland castle (n.)

the vagina.

[UK]Mercurius Democritus 31 May-7 June 37: Remembering his storming of Magy-land Castle, did what he could (for vindication of his Manhood) to get her to lay it [i.e. a baby] to him.