E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bailey, as if he actually believed it, gives the etymology of this word Duns stable; adding Duns or Dunus was a robber in the reign of Henry I., who made it dangerous for travellers to pass that way. (Dunes or duns tacell, our tablei.e. the table-land or flat of the hills.)
Downright Dunstable. (See DOWNRIGHT.)
Plain as the road to Dunstable; or, as Shakespeare says, Plain as way to parish church. The road leading to Dunstable is the confluence of many leading to London, but the play is on the word dunce.