When someone defecates onto another person's butt, and stuffs the pieces into their butthole.
They wanted to try the Milk Dud Stuffer, so they shat on the other person's butthole and stuffed the small pieces of poop into it.
When you and your mates are smoking out of a bong, and your mate takes the really long hit "milk that bitch" is encouraging words to keep going
Joe: *taking his hit*
Dherron: milk that shit