A beer placed in a pocket for future drinkage. Created by Mike O'Leary while hiking through Baird's Creek, WI

A beer (bottle or can) typically stored in one's pocket while the individual already has one open for consumption. The pocket beer is stored for use in the near future.

A beer that was purchased in the past for future consumption, even though you currently were currently consuming other beverages, but knowing that when said cocktails were finished, he would want another beverage.