"Hawkeye would walk confidently into a pro shop, smile, comment upon the nice condition of the course, explain that he was just passing through and that he was Joe, Dave or Jack Somebody, the pro from Dover. This resulted, about eight times out of ten, in an invitation to play for free. If forced into conversation, he became the pro from Dover, New Hampshire, Massachusetts, New Jersey, England, Ohio, Delaware, Tennessee, or Dover-Foxcroft, Maine, whichever seemed safest."
Later on in the book, when Hawkeye and fellow surgeon Trapper are called from Korea to Tokyo to perform surgery on a congressman's son, the following exchange takes place:
"'All right,' Trapper said. 'Somebody trot out the latest pictures of this kid with the shell fragment in his chest.'
No one moved.
'Snap it up!' yelled Hawkeye. 'We're the pros from Dover, and the last pictures we saw must be forty-eight hours old by now.'"
This latter exchange is repeated in the 1970 movie, but the term pros from Dover is not explained in the script. People who had seen the movie, but not read the book, started using the phrase to mean outside experts/consultants without understanding that Hawkeye was using the term facetiously, referring to an old con he used to run.