The Bodleian Library apparently acquired the manuscript in 1927 when it bought the papers of John Jamieson, a Scottish lexicographer who worked in the 19th century, but did not realize what it had, The Scotsman reported Sunday. Susan Rennie, a Scottish language expert, found it while researching a biography of Jamieson.
Experts have confirmed the manuscript was written by Boswell.
Boswell apparently planned to do his own Scottish dictionary modeled on his friend Johnson's English dictionary.
"He advised me to complete a dictionary of words peculiar to Scotland, of which I showed him a specimen," Boswell wrote.
Johnson, who traveled with Boswell to the Hebrides, sometimes teased the Edinburgh-born lawyer about being Scottish. Boswell was both self-conscious about his heritage and proud of it.
"The Scottish language is being lost every day, and in a short time will become quite unintelligible," Boswell wrote in an account of a trip to the Netherlands. "To me, who have the true patriotic soul of an old Scotsman, that would seem a pity."