pre-drinking
n.— «Because of the high cost of drinks in bars, those who can legally drink participate in what is known as “pre-drinking,” where they down as much as they can before they go out.» —“Mel’s Hell” by Georgie Binks CBC News (Canada) Aug. 11, 2006. (source: Double-Tongued Dictionary)
It is usually called “pregaming” by the hip kids:
http://abcnews.go.com/GMA/Health/story?id=1085909&page=1
Nice, thanks, I’ve made a cite.