Da liegt der Hund begraben

Literally: That's where the dog is buried

Example: “Niemand prüft die Rechnungen, und da liegt der Hund begraben.” — Nobody checks the invoices, and that's the heart of the matter.

English equivalent: The heart of the matter

Origin: No theory is definitively confirmed, and several compete. One reaches for Middle High German, where Hund is claimed to have carried a secondary sense close to "treasure" or "valuable possession" — so "there lies the Hund" originally meant something valuable was buried at that spot, before the word's more specialized old sense was lost and Hund was reinterpreted literally as a dog. A second, related theory points to old strongboxes and treasure chests, often decorated on the inside with the image of a fierce guard dog meant to symbolically ward off thieves — knowing where the chest was buried meant, in a sense, knowing where "the dog" was too. A third, much more local theory traces the phrase to the small Thuringian village of Winterstein, which still has a memorial stone commemorating a dog once said to have lived and died there. None of the three explanations is conclusively documented over the others.