Very blunt, plain speaking. The present town of Dunstable is at the foot of the Chiltern Hills, in Bedfordshire. There was somewhere about the same site a Roman station called Magionium or Magintum, utterly destroyed by the Danes, and afterwards overgrown by trees. Henry I. founded the present town, and built there a palace and priory.
“If this is not plain speaking, there is no such place as downright Dunstable.” —Sir W. Scott: Redgauntlet, chap. xvii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894