(1 syl.). The native language of the West Highlanders of Scotland, who are of Irish origin. It is a variant of Irish. Applied by the Scotch Lowlanders to the Highland dialect of Gaelic. In the eighteenth century Scotch was often called Erse, without distinction of Highland and Lowland; and Irish was spoken of as Irish Gaelic. The practice now is to limit the word Erse to Irish, and Gaelic to Scotch Highlanders.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894