The Roman wall between the friths of the Clyde and Forth, so called from the first person who leaped over it after the Romans left Britain.
“This wall defended the Britons for a time, but the Scots and Picts
assembled themselves in great numbers, and climbed over it... A man
named Grahame is said to have been the first soldier who got over, and
the common people still call the remains of the wall`Grahame's Dike.'”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894