Green’s Dictionary of Slang

mumble-peg n.

also mumbledepeg
[SE mumble-the-peg; mumblety-peg, a game in which each player in turn throws a knife from a series of positions, continuing until they fail to make the blade stick in the ground]

the vagina.

[UK] R. Brome New Academy II i: And she ha’ not good box and steel, I shall so grull her, And then at Mumbledepeg I will so firk her.
[UK]Farmer & Henley Sl. and Its Analogues.
[US]B. Jackson Get Your Ass in the Water (1974) 148: There was Mumblepeg with a dick like a table peg, / her and her partner, Anna Funk.