E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The same as Scythian in etymology; the root of both is Sct. The Greeks had no c, and would change t into th, making the root skth, and by adding a phonetic vowel we get Skuth-ai (Scythians), and Skoth-ai (Scoths). The Welsh disliked s at the beginning of a word, and would change it to ys; they would also changed c or k to g, and th to d; whence the Welsh root would be Ysgd, and Skuth or Skoth would become ysgod. Once more, the Saxons would cut off the Welsh y, and change the g back again to c, and the d to t, converting the Ysgod to Scot.
N.B. Before the third century Scotland was called Caledonia or Alban.