des Pudels Kern
Literally, ‘the poodle’s core’. A quote from Faust, Part One: a black poodle follows Faust home and then transforms into a wandering scholar (but is actually Mephistopheles in disguise). Faust exclaims “Das war also des Pudels Kern” (“So that was the poodle’s core, i.e. so that was what was going on inside the poodle”).