Origins
The word 'further' descends from Old English 'furþor,' the comparative form of 'forþ' (forth, forward). Its Proto-Germanic ancestor *furþer- was built from *fur- (forward) with the comparative suffix *-þer-, meaning 'more forward.' The ultimate root is PIE *per-, one of the most fundamental spatial and directional roots in the language family, meaning 'forward,' 'through,' or 'in front of.'
The PIE root *per- generated a vast family of English words, mostly denoting forward motion or position. From the Germanic branch: 'forth' (forward), 'further' (more forward), 'first' (most forward, the superlative), 'for' (before, on behalf of — originally a spatial 'in front of'), 'fore' (in front), 'before' (by the front), 'far' (forward by a great distance), 'from' (away from a forward position), and 'forward' itself. From Latin 'pro-' and 'per-': 'proceed,' 'produce,' 'progress,' 'project,' 'promote,' 'provide,' 'provoke,' 'permit,' 'perform,' 'perfect,' 'persist,' and dozens more. From Greek 'pro-': 'problem,' 'program,' 'prophet,' 'prologue,' and 'prototype.'
The distinction between 'further' and 'farther' is one of English's most frequently asked usage questions. The traditional prescription, codified by usage guides in the twentieth century, is that 'farther' should be used for physical distance ('we walked farther') while 'further' should be used for figurative or abstract extension ('further discussion,' 'further evidence'). In practice, the two words have been used interchangeably for most of English's history. 'Further' is the older form; 'farther' was created in Middle English as a variant influenced by 'far' (which is actually from a different PIE root, though the words became conflated). The careful distinction is a modern regularization, not a historical rule.
Old English Period
As a verb, 'to further' means 'to advance or promote something': 'to further one's career,' 'to further the cause of justice,' 'to further scientific research.' This verbal use dates from Old English and preserves the original comparative sense: to move something more forward, to advance it along its path.
The word 'furthermore' (in addition, moreover) combines 'further' with 'more,' creating a double comparative — literally 'more-forward-more.' This redundancy, common in English transitional phrases, serves a rhetorical rather than logical purpose: it signals emphatically that additional information follows.
The related word 'first' deserves attention as the superlative of the same root. Old English 'fyrst' (first, earliest) is from Proto-Germanic *furistaz, the superlative of *fur- (forward). 'First' literally means 'the most forward' — the one at the front of a sequence. The connection between 'first' and 'further' is thus grammatical: first is the superlative, further is the comparative, and forth is the positive degree of the same root concept — forward motion.