Cowboy beer is beer that has been through at least one set of kidneys. A cup of beer piss, to be blunt. Thrifty cowpokes have long known that one can fill up their mug on the cheap from an unflushed toilet or urinal in a beer-sloshed country/western bar. Odds are good that drinking this "cowboy beer" will make the drinker tipsy. Studies say that alcohol potency is increased when passed through a set of kidneys, so grab a mug, buddy!
Trey left his wallet at home, so he drank cowboy beer all night at the honky bar, listening to 'Achy Breaky Heart' on the jukebox.