The person(s) involve in the theft of beer from one party to take back to the party of their own. Once a beer ninja accepts the mission he must complete under any means neccessary. This includes but is not limited to 1)stealing from a friends party, 2)jumping fences, 3) wearing camoflage, 4)kidnapping people, 5) attacking from the sea, and 6)sending in spies to distract parties away from the mass of beer.
Chris: We are out of beer dude
Nick: No shit? Guy down the street is having a party.
Chris: Lets take his beer.
Nick: Its on. Beer Ninja style.