Green’s Dictionary of Slang

naggie n.

also naggy
[nag n. (1) + dimin. -ie; note SE naggie, a pony, which can be ‘ridden’]

the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Farmer Vocabula Amatoria (1966) 35: Berlingot, m. 1. The female pudendum; ‘the naggie’.