This is a jargon word of those groups of enthusiasts who go from place to place re-enacting historic battles. It’s American in origin, but has also become known in other countries. Re-enactors are deadly serious about getting the details of uniform and equipment correct. They are dismissive of the people they call farbs who come for the fun but who don’t make the effort to get things right, the sort of people who will mix up items of uniform or carry a mobile phone or wear sunglasses.
The term dates from the 1960s; an explanation for its origin was given in July 1986 in a re-enactors’ magazine, the Camp Chase Gazette, in which an early group leader, George Gorman, was said to have formed it from the beginning of “Far be it from me to criticise inauthentic uniforms ...”.
But the memories of early re-enactors asserts the word instead derives from German farbe, colour, because inauthentic re-enactors were over-colourful compared with the dull blues, greys or browns of the real Civil War uniforms that were the principal concern of American re-enactors at the time the word was coined. Farb was created by Gerry Rolph, a German teacher who led one of the early bands. The adjective farby is also known and may indeed have been the original form.
[I’m grateful to Jonah Begone of the Camp Chase Gazette for setting out the stories about farb on his Web site, and to Robert Fineberg, who stimulated my interest in the word.]