An Etymological Dictionary of the German Language/Annotated/fürder
fürder, adverb, ‘further,’ from Middle High German vürder, Old High German furdir, adverb, ‘further in front, further on, away’; apparently an oblique form of the comparative neuter, like Gothic faurþis, ‘formerly,’ from fort, Gothic *faurþ; Anglo-Saxon furþor, adverb, ‘forward, further, more distantly’ (Gothic *faúrþôs), English further. See fort.