E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 Romar 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.