From Longman Dictionary of Contemporary Englishdufferduf‧fer /ˈdʌfə $ -ər/ noun [countable] informal STUPID/NOT SENSIBLEsomeone who is stupid or not very good at something
Examples from the Corpus
duffer• Oliver says he is a duffer, but keeps him on because he is so helpless and lonesome.• The agent's an awful duffer when it comes to dates.• By refraining I have no doubt I shall receive the silent blessings of the multitude of duffers among whom I find myself.• I was never your prototypical duffer.• I think a lot of people feel vaguely when they see two duffers marrying that the duffers must prefer it that way.Origin duffer (1700-1800) Perhaps from Scottish English doofart “stupid person” 