Dudman and Ramhead
When Dudman and Ramhead meet. Never. Dudman and Ramhead (now spelt Ramehead) are two forelands on the Cornish coast, about twenty miles asunder. (See Never.)
“Make yourself scarce! depart! vanish! or we'll have you summoned before the mayor of Halgaver, and that before Dudman and Ramhead meet.” —Scott: Kenilworth, iv.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894