I believe it was Cory (beaker) who coined this phrase.
Muffin, as I posted in the "New Members" thread, means to take something completely off-course, or to invoke silliness. For example, if you take a thread from its original subject into something completely different (and usually very silly), you've "Muffined" a thread. Or, if you wish to draw some one into something silly, you could say something to the effect of "have a muffin!

".
So, quit your whinnin' and have a freakin' Muffin already!
-Matt