Farb

Various explanations of the origin are given:

That it is a contraction of the phrase "'Far be' it for me to criticize anyone, but..."
That it comes from the German word Farbe ("color"). (Many fabrics dyed with modern dyes are "too colorful" to be authentic, by comparison with their historical originals.)
That it stems from the rating of a reenactor's portrayal as "'Far below'" the standard.
There exists a letter dated 1 April 1863 from an A.R. Crawford in the 76th Illinois Infantry, Co D, that uses the phrase, "fallacious accoutrements & reprehensible baggage," in description of six children posing in phony military gear during a sham reenactment that took place during the actual Civil War. Many point to this phrase as the origin of the word, citing "farb" as an acronym.

Full definition of farb

(plural farbs)

(US) A historical reenactor (especially a US civil war reenactor) whose efforts at a historically accurate portrayal are, in the opinion of the speaker, inadequate. (For example, wearing a modern wristwatch with period costume.) The opposite of farb is "hard-core" (or hardcore), someone who is, in the opinion of the speaker, an "authenticity fanatic".