Summary

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

But the coast regions of Scotland, lying right opposite to the Hebrides, had already a greatly preponderant Celtic-speaking population, and there was thus no bulwark that could defend the Norn of the Hebrides against the steadily continued pressure of the Celtic of West Scotland, which at last overwhelmed it.
The Hebrides, also, lay much farther away from Norway than Orkney and Shetland, and were separated from Norway at an earlier date than these latter Island-groups, that is, immediately after King Håkon Håkonsson’s ill-starred expedition to the West of Scotland.
Source: Wikisource

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

Where Shetlandic words have meanings that are partly Northern, partly English-Scottish, all the meanings are usually given except those that have come in, or must be considered to have come in, very recently from standard English. On the other hand, a form such as “goul, gowl”, vb., to cry, howl, has not been entered, although it corresponds rather to O.N. gaula (goula) and No. gaula (goula) . In Lowland Scottish the word has the same form as in more recent Shetl., in Jam.: goul, gowl. But in Shetl. Norn “au, ou” regularly change to o (commonly with prefixed j: jo) , occasionally to u or ø.
Source: Wikisource

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

Celtic was spoken in Orkney and Shetland in ancient times, and long after Northmen had settled there. It was gradually displaced by Norse in proportion as the number and power of the immigrating Northmen increased, and not all at once. But, while the Norn, afterwards displaced by Lowland Scottish, has left behind it many and deep traces in the present dialect of the Islands, there remain now only very slight traces of the original Celtic.
Source: Wikisource

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