The history of the Scots language refers to how Anglic varieties spoken in parts of Scotland developed into modern Scots.
Origins
Speakers of Northumbrian Old English settled in south-eastern Scotland in the 7th century, at which time Cumbric was spoken in the south of Scotland up to the Forth-Clyde isthmus, and the possibly related Pictish was spoken further north. At the same time Gaelic speakers began to spread from the Western Coast of Scotland north of the Clyde into the east. Over the next five hundred years with the founding of Scotland and spread of Christianity across the north of Britain by the Columban Church the Gaelic language slowly moved eastwards and southwards across the lowlands. When Northumbrian lands were incorporated into Scotland in the 11th century Gaelic became the prestige language there and had some influence, but the south east remained largely English speaking. In the far north, Viking incursions brought Old Norse speakers into Caithness, Orkney, and Shetland.
Scholars of the language generally use the following chronology: Northumbrian Old English to 1100 Pre-literary Scots to 1375 Early Scots to 1450 Middle Scots to 1700 Modern Scots 1700 onwards
Pre-literary Scots
The nature of early forms of the language are obscure due to Viking plundering and destruction, Edward I of England's removal of the national records and their subsequent loss, the destruction of the monasteries in border warfare, and vandalism during the Reformation. It is difficult to assess whether Scots descends largely from the Northumbrian Old English of Lothian or the Anglo-Danish of Yorkshire introduced some four hundred years later, which would explain the Norse elements in Early Scots which are lacking in Northumbrian Old English. According to linguist Paul Johnston, Scots descends "from a radically restructured, Norse-influence Northumbrian going back to the Danelaw proper as much as from the original dialects of the Bernician settlers." Further Scandinavian influence could have come about through Scotland's trade contacts with Norway. Current insights into pre-literary Scots stem largely from place-names, archaeology, and a few words in Latin documents.
Early Scots
Northumbrian Old English had been established in south-eastern Scotland as far as the River Forth by the 7th century. It remained largely confined to this area until the 13th century, continuing in common use while Scottish Gaelic was the court language until displaced by Norman French in the early 12th century. English then spread further into Scotland via the burgh.
After the 12th century early northern Middle English began to spread north and eastwards. It was from this dialect that Early Scots, known to its speakers as "English" (Inglis), began to develop, which is why in the late 12th century Adam of Dryburgh described his locality as "in the land of the English in the Kingdom of the Scots" and why the early 13th century author of de Situ Albanie thought that the Firth of Forth "divides the kingdoms of the Scots and of the English".
Most of the evidence suggests that English spread further into Scotland via the burghs, proto-urban institutions which were first established by King David I. Incoming burghers were mainly English (especially from Northumbria and the Earldom of Huntingdon), Flemish and French. Although the military aristocracy employed French and Gaelic, these small urban communities appear to have been using English as something more than a lingua franca by the end of the 13th century. The growth in prestige of English in the 14th century, and the complementary decline of French in Scotland's Royal Court, made English the prestige language of most of eastern Scotland.
Divergence from Northumbrian Middle English was influenced by the Norse of Scandinavian-influenced Middle English-speaking immigrants from the North and Midlands of England during the 12th and 13th centuries, Dutch and Middle Low German through trade and immigration from the low countries, and Romance via ecclesiastical and legal Latin, Norman and later Parisian French due to the Auld Alliance. Some loan words entered the language resulting from contact with Scottish Gaelic, often for geographical features such as ben, glen, crag, loch and strath; however, there are several others like bog from bog (moist or damp), twig (catch on) from tuig (understand), galore (lots of) from gu leòr (plenty), boose or buss from bus (mouth), and whisky from uisge-beatha (water of life).