Urban Dictionary (I don’t recommend going there) defines beer cow as:
A technique used primarily in convenience stores where a person imitates a cow at the cash register when they don’t have enough cash to pay for what they’re buying.
I’ve never done this, surprisingly, but I’m assuming one person would be doing this to distract the cashier while the other leaves with the beer. Or, the cashier would just think it was sooooooooo funny he would just give the beer to those hilarious drunks.
Perhaps Timman (or the younger and not quite as famous TimBro) can do a test run of the latter part of this technique. Ha ha. Hoo hoo. Hee.