Green’s Dictionary of Slang

1. the vagina.

1984

Partridge DSUE (8th edn) 919/1: late C.19–20.

2. a general term of contempt; note mis-defined as an adj. in cit. 2001 [fig. use of sense 1].

1981		J. Sullivan ‘Big Brother’ Only Fools and Horses [TV script] I’m sorry, I don’t know the French for pranny!
1982		L. Cody Bad Company 45: She wasn’t much help. And neither was that other bunch of prannies.
2001		OnLine Dict. of Playground Sl. 🌐 pranny adj. A foolish person, a dur-brain: Used for example, as ‘fuck off, pranny!’. Obviously had very vague sexual undercurrent, as sounded a bit like ‘fanny’.