Following up from that entirely correct answer..According to Darby and Joan
Brewer's Dictionary of Phrase and Fable..... Darby and Joan are defined as 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. Further, Dar'bies is a word once used to describe handcuffs because either two prisoners or prisoner and police officer were linked together as Darby and Joan.
'Hark ye! Jem Clink will fetch you the darbies.''Sir W. Scott wrote in Peveril of the Peak.