(A) now means a person easily bamboozled, one of slow wit; but originally it meant one who cheated or bamboozled. To duff =to cheat. Persons who sell inferior goods as “great bargains,” under the pretence of their being smuggled, are duffers; so are hawkers generally. At the close of the eighteenth century passers of bad money were so called. Now the word is applied to persons taken in, and by artists to inferior pictures.
“Robinson a thorough duffer is.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894