E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Darby and Joan.
A loving, old-fashioned, virtuous couple. The names belong to a ballad written by Henry Woodfall, and the characters are those of John Darby, of Bartholomew Close, who died 1730, and his wife, As chaste as a picture cut in alabaster. You might sooner move a Scythian rock than shoot fire into her bosom. Woodfall served his apprenticeship to John Darby.
Perhaps some day or other we may be Darby and Joan.Lord Lytton.
The French equivalent is Cest St. Roch et son chien.