A dimwit, or a coward. The word divides as dowf + -art, not dow + fart — dowf is Scots for dull, and -art is the same pejorative ending hiding in braggart and dullard, which traces back to a Germanic root meaning bold. In use since 1719. The best part is what it became: duffer is probably just dowfart after the word quietly let out a few letters.