Stick it in a muffin (also phrased "place it in a muffin") basically means shut up, stop, yeild, do not continue...
Brittany: "Hey did I tell you about my date with Brad?!"
Lucille: "About a million times. Please, stop talking about Brad."
Brittany: "So, we went out to the movies, and-"
Lucille: "Gosh, Brittany, STICK IT IN A MUFFIN ALREADY!"