Wikipedia
Old Frisian was a West Germanic language spoken between the 8th and 16th centuries in the area between the Rhine and Weser on the European North Sea coast. The Frisian settlers on the coast of South Jutland (today's Northern Friesland) also spoke Old Frisian but no medieval texts of this area are known. The language of the earlier inhabitants of the region between the Zuiderzee and Ems River (the Frisians famously mentioned by Tacitus) is attested in only a few personal names and place-names. Old Frisian evolved into Middle Frisian, spoken from the 16th to the 19th century.
In the early Middle Ages, Frisia stretched from the area around Bruges, in what is now Belgium, to the Weser River, in northern Germany. At the time, the Frisian language was spoken along the entire southern North Sea coast. This region is referred to as Greater Frisia or Frisia Magna, and many of the areas within it still treasure their Frisian heritage. However, by 1300, their territory had been pushed back to the Zuiderzee (now the IJsselmeer), and the Frisian language survives along the coast only as a substrate.
The people from what are today northern Germany and Denmark who settled in England from about 400 onwards came from the same regions and spoke more or less the same language as the people who lived in Frisia (as medieval Friesland is usually called to distinguish it from the present-day regions with that name). Hence, a close relationship exists between Old Frisian and Old English.