Slinging human or animal faeces (poo) from an empty pringles tubes out of a moving vechile at somebody.
Ali P - Hey bro, I've been wanting to go prooping for a while should we?
Nathan - Yeh you poo in the pringles tube then
(Flings poo at passerby)
Ali P - Bro you just totatly prooped that guy bro
Valium is a brand name for diazepam, used to treat anxiety and generally calm people down. A benzodiazepine, it is quite addictive. Often combined with alcohol for people who want to get relaxed and drunk quickly.