Used by the Fat Lady as a Gryffindor password in sixth year (HBP12, HBP14). A kind of stew or porridge that historically was offered to the British monarch on coronation day by the lord of the manor of Addington, Surrey (NSOED)
Other passwords used in Harry's sixth year for Gryffindor Tower are: baubles (HBP15), toffee eclairs (HBP20), tapeworm (HBP23), and quid agis (Latin for "How do you do?") (HBP24).
According to the Food History Almanac the soup known as "Dilligrout" was first prepared for the coronation of Queen Matilda, the wife of William the Conqueror, in 1068. It was invented by a cook named Tezelin who so pleased the King that he gave him a manor in Addington, Surrey, as long as his family would continue to provide Dilligrout for royal coronations into the future. The tradition lasted 1000 years until 1821, when for some reason the royals stopped requesting it. Dilligrout was "compounded of almond milk, the brawn of capons, sugar and spices, chicken parboiled and chopped, and was called, also, ‘Le mess de gyron'."
Why was this the Gryffindor password? It could be the author's way of giving a clue about Draco Malfoy, who lived on a Manor in Surrey, but was plotting to kill Dumbledore at the time, and instead was hurting other students like Katie Bell or Ron Weasley. It could also be a reference to the Half-Blood Prince, whom Harry at one time believed was some kind of magical royalty, but in reality was Severus Snape born in rather lowly circumstances.