Euphemism for "bullshit" or "shit" interchangably in the Colonial vernacular. The appeal of the word "felgercarb" would seem to extend across the lines of culture and species, as Lucifer uses the term several times in "The Young Lords."
An alternate spelling, "feldercarb", can be found in the Writer's Guide, dated October, 2, 1978, which lists this as its definition:
"Cut through the feldergarb" - "Eliminate the bull"