Green’s Dictionary of Slang

PP9 v.

(UK prison) to hit someone with a large, heavy PP9 battered placed inside a sock.

http://www.city-journal.org Spring 🌐 It took me back to my early days in the prison where I worked, when I would be asked to see prisoners who had just been ‘PP-nined.’ [...] a PP9 was a large, heavy, square battery [...] , a prisoner would proceed to place the battery inside a sock, swing the sock as if it were a South American bola, and attack his enemies with the improvised weapon.