Thomas the Rhymer was so called from his prophecies, the most noted of which was the prediction of the death of Alexander III. of Scotland, made to the Earl of March in the Castle of Dunbar the day before it occurred. It is recorded in the Scotichronicon of Fordun. (1430.) (See Rhymer.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894