We have not a single Scots dictionary. Really, that is amazing. I believe there is not another language in Europe … of which there is not some sort of lexicon.
(Boswell in Holland, p. 160)

Boswell’s Scots dictionary, had he completed and published it, would have been the first work of its kind. In the 1760s, when Boswell embarked on his project, Scots lexicography had two dominant strands: glossaries to literary texts, either for Older Scots (such as the celebrated glossary to Gavin Douglas by Thomas Ruddiman) or more modern works (such as Allan Ramsay’s glossaries to his own poems); and proscriptive lists of Scotticisms, aimed at those who wished to eradicate Scots usages from spoken or written English. Boswell began his work only a few years after David Hume’s list of Scotticisms was reprinted and widely circulated in the Scots Magazine. Although Ruddiman had included some modern Scots usages in his glossary, there had been no systematic attempt before Boswell to compile a dictionary of modern Scots. Later in the 18th century, many similar plans were begun, including one by Boswell’s friend, Lord Hailes, but it was not until 1808, with the publication of Jamieson’s Dictionary, that a complete work was produced.

One of the many interesting aspects of Boswell’s manuscript is that he displays none of the proscriptive attitude towards Scots with which he is sometimes associated. It is a dictionary of Scots, not of Scotticisms. Although Boswell gives English equivalents for his Scots headwords, they are listed (in his worked-up specimen) alongside other equivalents in Latin and in modern European languages, such as French, German, Italian, and Dutch, with which Boswell came into contact on his travels through Europe. Because Boswell was especially keen to practise his French at this period, many of the specimen entries are actually defined in French, leading to the curious circumstance that Boswell’s draft dictionary is therefore the first (and, to my knowledge, the only) Scots-French dictionary. The next European Scots dictionary (this time from Scots to German) was published over sixty years later, in 1826, by Robert Mothesby, largely in response to European interest in the novels of Sir Walter Scott.

Further reading:
M. Dareau and I. Macleod, ‘Dictionaries of Scots’, in The Oxford History of English Lexicography, ed. by A. P. Cowie, 2 vols (Oxford: OUP, 2009), I, 302–25
J.D. McClure ‘Glossaries and Scotticisms: Lexicography in the Eighteenth Century’, in Scotland in Definition: A History of Scottish Dictionaries, ed. by I. Macleod and J.D. McClure (Edinburgh: John Donald, 2012)