Basically defined as 'a simpleminded individual with few social skills and even less intellectual capacity'. In use very much the London equivalent of Pranny. For example, see verse from a song by Ian Dury (1977) called Billericay Dickie:

I'd redezvous with Janet
Quite near the Isle Of Thanet
She looked more like a gannet.
She wasn't 'alf a prannet.
Her mother tried to ban it
Her father helped me plan it
and when I captured Janet
She bruised her Pomegranite

(ed: I wonder if anyone would like to send me an MP3 of this? sounds interesting!)