The Collaborative International Dictionary of English v.0.48:

Duffer \Duf"fer\, n.
   1. A peddler or hawker, especially of cheap, flashy articles,
      as sham jewelry; hence, a sham or cheat. [Slang, Eng.]
      --Halliwell.
      [1913 Webster]

   2. A stupid, awkward, inefficient person.[Slang]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duffer \Duf"fer\, n.
   1. (Mining) See Shicer.
      [Webster 1913 Suppl.]

   2. (Zool.) Any common domestic pigeon.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Duffer \Duf"fer\, n.
   One who duffs cattle, etc. [Australia]

         Unluckily, cattle stealers are by no means so rare as
         would be desirable; they are locally known as duffers.
                                                  --Baden-Powell.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

duffer
    n 1: an incompetent or clumsy person; "as a golfer he was only a
         duffer"

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "duffer":
   arab, blockhead, blunderer, boob, bungler, butterfingers, clumsy,
   dimwit, dolt, dope, dub, dumbbell, foozler, foul-up, fumble-fist,
   goof, goofball, goofer, hawker, hick, higgler, huckster, idiot,
   ignoramus, incompetent, lummox, lump, monger, muff, muffer,
   numskull, oaf, ox, roadman, rube, slob, stumblebum, stumblebunny,
   vendor