"Duffer" is a colloquial or slang term for a mediocre or poor golfer. The term can be derogatory, but often isn't. If you say, for example, that "most of the golfers who play this golf course are duffers," there's nothing really insulting about that. After all, for most public golf courses, the majority of golfers on it will be mediocre to poor players (higher handicappers, in other words - recreational golfers, most of whom never break 100). And there's nothing wrong with that!
However, if you call a good golfer - a low-handicapper - a duffer, then the term becomes derogatory. And woe unto him who makes the mistake of referring to a golf pro as a "duffer."
Duffer is somewhat synonymous with "hacker" in that they both apply to weaker players. But hacker is the stronger, more derogatory term.
The Merriam-Webster dictionary's second definition for "duffer" is "an incompetent, ineffectual, or clumsy person; especially: a mediocre golfer." Non-golf meanings of the term (again, citing Merriam-Webster) include "a peddler, especially of cheap flashy articles;" and "something counterfeit or worthless."
In golf, the term's origin is likely aligned with that of "duff," which is sometimes used to indicate a mis-hit ("Bob duffed his chip shot").
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Also Known As: Hacker, weekend hacker
Examples: "Bob's a duffer on the golf course, but he has fun."