An Etymological Dictionary of the German Language/Fürst
Fürst, m., ‘‘sovereign, chief, prince,’ from MidHG. vürste, m., ‘the highest, most distinguished, ruler, prince,’ OHG. furisto, OSax. furisto, Du. vorst, ‘prince’; like Herr, simply a Ger. form. Just as Herr is orig. a compar. of hehr, so is Fürst prop. a superlat. meaning ‘first’; comp. OHG. furist, AS. fyrst, E. first, OIc. fyrstr; Goth. *faúrista is wanting; the corresponding compar. is OHG. furiro, ‘the former, preferable,’ OIc. fyrre, ‘former.’ The usual OSax. and AS. word for ‘first’ is formo, forma, with the suffix ma (Goth. fruma); from Aryan pṛ like Gr. πρόμος, Sans. pûrva-s, OSlov. prŭvŭ, Lith. pírmas, ‘first.’ It is evident that also vor, für, fort, &c., are derivatives of this Aryan root pṛ.