An an alcoholic drink made up of a can of beer and a shot of whiskey. Like a Boilermaker, but the drink is made by taking a swig from the can of beer, and filling the resulting space in the can with whiskey (Bourbon is prefered). Named as a tribute to Walter Matthau's character Morris Buttermaker in the Bad New Bears, who can be seen making them throughout the movie. A great tailgating drink.
Hand me the Jim Beam and hold out your beer, Buttermakers all around.