to mix an alcoholic drink with juice; one cup of alcohol with a straw, and another cup of juice with a straw. Drink from both straws at the same time, and you'll look like a walrus, hence the name of the drank.
Rick: "Dude, would you like to walrus?"
Joe: "Wait, you mean like have oral sex or something?"
Rick: "NO, man, the DRINK!"
Joe: "Oh, like the one with the straws?"
Rick: "Damn, Ana's walrusing the hell out of those cups right now!
Joe: "Move the fuck over Ana, I wanna walrus!"