The act of one individual exposing their breasts and laying (back down) on a keg of beer. At this point, a second party mounts, much as if to do a keg stand, but in lieu of beer, the second party chugs milk out of the exposed mammary gland(s). And if a third party enters to perform fellatio on the mam' stander - then - bonus.
Kage's totally gonna do a ten minute mam' stand on Bridget's Bailey's-lactating tits. Tailgates start at 8:30, damnit.