des Pudels Kern
Literally, “the poodle's core”. The phrase is taken from a scene in Faust, Part One during which a black poodle follows Faust home and transforms into a wandering scholar, who is actually Mephistopheles in disguise. Faust exclaims: "Das war also des Pudels Kern!" (“So that was the poodle's core!”); that is, "So that's what was going on inside the poodle!".