Green’s Dictionary of Slang

a fool, an idiot.

1977		Ian Dury ‘Billericay Dickie’ 🎵 I’d rendezvous with Janet / Quite near the Isle Of Thanet / She bruised her Pomegranite / She looked more like a gannet / She wasn’t ’alf a prannet.
1996		F. Wheen Lord Gnome’s Literary Companion 103: So the prannet gets to say things such as [etc.].
2001		OnLine Dict. of Playground Sl. 🌐 Prannet A simpleminded individual with few social skills and even less intellectual capacity. In use very much the London equivalent of Pranny.