Oxford English Dictionary's commentary on Scotch (as short form of Scottish) and Scots entries; the etymology portion of Scotch:
Contracted var. of SCOTTISH.Thanks to James for posting this on the APG list.
The three forms of the adj., Scotch, Scottish, Scots, are still current, with some difference in use, which, however, is somewhat unsettled. Down to the middle of the 16th c. the only form used in southern English was Scottish; but in the dialect of Scotland (and in that of the north of England in the 14th and 15th c.) the form was Scottis (cf. Inglis = English), subsequently contracted to Scots. So far as our quotations show, the contraction of Scottish into Scotch is not recorded before 1570 (in the compound Scotchman), though the colloquial pronunciation which it represents may well be much older; instances of Scotch cap, Scotch jig occur in 1591-99, but the adj. did not become common in literature until the second half of the 17th c. From that time until the 19th c. Scotch has been the prevailing form in England, though Scottish has always been in use as a more formal synonym. In Scotland, the authors who wrote in dialect (down to Ramsay and Fergusson early in the 18th c.) used Scots, while those who anglicized adopted the form Scottish. But before the end of the 18th c. Scotch had been adopted into the northern vernacular; it is used regularly by Burns, and subsequently by Scott; still later, it appears even in official language in the title of the 'Scotch Education Office.' Since the mid 19th c. there has been in Scotland a growing tendency to discard this form altogether, Scottish, or less frequently Scots, being substituted. At the beginning of the 20th c., while in England Scotch was the ordinary colloquial word, the literary usage prefered Scottish in applications relating to the nation or the country at large or its institutions or characteristics. Thus it was usual to speak of 'Scottish literature,' 'Scottish history,' 'the Scottish character,' 'a Scottish lawyer,' 'the Scottish border.' On the other hand, it would have sounded affected to say 'a Scottish girl,' 'a Scottish gardener.' Although 'the Scottish dialect' is now the usual designation, it is seldom that Scottish is used as a n. instead of Scotch. Recent usage favours Scots in 'Scots law,' and it is now almost universal in historical references to money, as 'a pound Scots.'
In the 20th c. the word Scotch has been falling into disuse in England as well as in Scotland, out of deference to the Scotsman's supposed dislike of it; except for certain fixed collocations, (such as 'Scotch mist,' 'Scotch whisky') Scottish (less frequently Scots) is now the usual adjective, and to designate the inhabitants of Scotland the pl. n. Scots is preferred (see Gowers/Fowler Mod. Eng. Usage (1965)).
Related link - Scotch-Irish Central: http://www.scotch-irishcentral.org/
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