Origins
The English word 'eight' descends from Old English 'eahta,' from Proto-Germanic *ahtō, from Proto-Indo-European *oḱtṓw. This numeral shows exceptional uniformity across the Indo-European family: Latin 'octō,' Greek 'oktṓ,' Sanskrit 'aṣṭā́,' Old Irish 'ocht,' Lithuanian 'aštuonì,' and Old Church Slavonic 'osmĭ' are all transparently related, differing mainly in their treatment of the medial consonant cluster.
One of the most intriguing aspects of the PIE form *oḱtṓw is its apparent morphological structure. Several prominent linguists, including Émile Benveniste, have analyzed it as a dual form — specifically as *h₃eḱt-h₃ow, which could be interpreted as 'two fours' or 'a pair of fours.' If this analysis is correct, it suggests that PIE speakers conceptualized eight not as an independent quantity but as two groups of four, possibly reflecting a counting system that used four as a sub-base (a 'quaternary' element within the decimal system). The number nine shows similar evidence of being derived from 'new' (*h₁néwn̥, possibly 'the new number' after the completion of two fours), which would support a system where the basic counting unit was a group of four (matching the four fingers excluding the thumb).
The consonant cluster in the middle of the word has had a dramatic history in English. PIE *ḱt became Proto-Germanic *ht (via Grimm's Law, where *ḱ became *h and *t was preserved). In Old English, this *ht was still pronounced as a voiceless velar fricative /x/ followed by /t/ — similar to the 'cht' cluster in German 'acht' or Scottish English 'loch' followed by a /t/. During the Middle English period, the fricative /x/ gradually weakened and was eventually lost entirely in standard English, but the spelling 'gh' was retained, creating one of English's most iconic silent-letter combinations. The same process affected 'night' (Old English 'niht'), 'light' (Old English 'lēoht'), 'daughter' (Old English 'dohtor'), and 'thought' (Old English 'þōht'). German and Dutch, by contrast, preserved the fricative: German 'acht' /axt/ and Dutch 'acht' /ɑxt/.
Old English Period
The vowel development is also noteworthy. Old English 'eahta' had a diphthong 'ea' followed by the fricative. As the fricative weakened, the preceding vowel underwent compensatory changes, eventually merging with the long 'a' that later underwent the Great Vowel Shift to become /eɪ/. The modern pronunciation /eɪt/ thus represents the collapse of what was originally a three-syllable or heavy-diphthong-plus-consonant sequence.
Through Latin 'octō,' the 'eight' root entered English in numerous learned borrowings. 'October' was originally the eighth month of the Roman calendar (which began in March); like September, November, and December, it kept its numerical name even after the calendar was reformed. 'Octave' (a group of eight, especially in music) comes from Latin 'octāva.' 'Octopus' is from Greek 'oktṓpous' (eight-footed), with the plural 'octopodes' in Greek, though English speakers have created the analogical plurals 'octopuses' and the folk-etymological 'octopi' (treating the Greek word as if it were Latin second declension). 'Octane,' the eight-carbon hydrocarbon, takes its name from the same root.
The number eight has geometric significance as the number of vertices of a cube and the number of faces of an octahedron (one of the five Platonic solids). In East Asian cultures, eight is considered extremely lucky — in Chinese, 'bā' (eight) sounds similar to 'fā' (to prosper), making it the most auspicious number. The Beijing Olympics opened on 8/8/08 at 8:08 PM. This cultural significance is unrelated to the Indo-European etymology but illustrates how different civilizations can invest the same abstract quantity with vastly different symbolic meanings.