Is it farther or further?
mid-15c., "fortified place, stronghold," from Middle French fort, from noun use in Old French of fort (adj.) "strong, fortified" (10c.), from Latin fortis "strong, mighty, firm, steadfast," from Old Latin forctus, possibly from PIE root *bheregh- "high, elevated," with derivatives referring to hills and hill-forts (cf. Sanskrit brmhati "strengthens, elevates," Old High German berg "hill;" see barrow (n.2)).