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