A beer (usually in a bottle) that needs to be consumed, but before it's finished, a cab is hailed. Since leaving a beer by itself is a form of alcohol abuse, you must sneak the beer into the cab and consume it while en route to your destination. This works well in conjunction with a "chicago", where a beer is passed around in order to consume it quickly. A "Cab Beer" is most easily performed in the back of a van-taxi, although certain species(ninja, pirate, lumberjack, chuck norris) can pull this off in a standard taxi. People who do "Cab Beer" should be held in the highest regard, for it is a sign of good taste and character.
(at a bar)
me: let's go, the show already started!
(both notice at the full opened beer bottle on the table)
me: shoot, isn't that our beer?
her: We'll knock it out on the way to the show, it's a cab beer!