E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who importunes for payment of a bill (Anglo-Saxon, dunan, to din or clamour). The tradition is that it refers to Joe Dun, a famous bailiff of Lincoln in the reign of Henry VII. The British Apollo says he was so active and dexterous in collecting bad debts that when anyone became slow to pay the neighbours used to say to the creditors, Dun him (send Dun after him).
An Universitie dunne is an inferior creditor of some ten shillings or downewards, contracted for horse-hire, or perchance drinke, too weake to be put in suite.Bishop Earle: Microcosmographia (16011695).
Squire Dun. The hangman between Richard Brandin and Jack Ketch.