Is it farther or further?
1650s, past participle adjective from alternative past participle (originally in Scottish legal use) of prove (v).
late 12c., pruven, proven "to try, test; evaluate; demonstrate," from Old French prover, pruver "show; convince; put to the test" (11c., Modern French prouver), from Latin probare "to make good; esteem, represent as good; make credible, show, demonstrate; test, inspect; judge by trial" (source also of Spanish probar, Italian probare), from probus "worthy, good, upright, virtuous," from PIE *pro-bhwo- "being in front," from *pro-, extended form of root *per- (1) "forward, through" (see per), + root *bhu- "to be" (cf. Latin fui "I have been," futurus "about to be;" Old English beon "to be;" see be). Related: Proved; proven; proving.