Wikipedia informs us that this punning nickname was coined by Robert Southey, who was a friend of Telford's. It continues: "Telford's reputation as a man of letters may have preceded his fame as an engineer: he had published poetry between 1779 and 1784, and an account of a tour of Scotland with Southey. His will left bequests to Southey (who would later write Telford's biography), the poet Thomas Campbell (1777–1844) and to the publishers of the Edinburgh Encyclopædia (to which he had been a contributor)."
Robert Southey was Poet Laureate from 1813 until his death in 1843. (Telford died in 1834.)
© Haydn Thompson 2020