A muffkin is directly connected to its original meaning of a blumpkin. A muffkin, however, is the act of getting an oral service while taking a dump. The only difference is the guy is preforming the service hence the name "Muff"kin.
Brian: "Dude, I was giving this spanish chick a muffkin last night right? And a little bit got on my chin!"
Joe: "Sweet! Did you take pictures?"