It is a variety of Standard Danish (rigsmål, rigsdansk) influenced by the surrounding German language in relation to prosody, syntax and morphology, used by the Danish minority in Southern Schleswig.
Originally Southern Jutlandic was spoken in most parts of the area (in the variants of Angel Danish and Mellemslesvigsk).
On the western coast, North Frisian was also spoken.
After the language shift in the 18th, 19th and 20th centuries, most of the Danish and North Frisian dialects were replaced by Low and Standard German.
A similar phenomenon is Gøtudanskt on the Faroe Islands.