des Pudels Kern

Etymology
lit. 'the poodle's core'. The phrase is taken from a scene in  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!"; that is, "So that's what was going on inside the poodle!".

Noun

 * 1) the crux of the matter, the gist of the matter, the heart of the matter