Origins
The English verb 'know' descends from Old English 'cnāwan' (to know, perceive, recognize), from Proto-Germanic *knēaną, from the Proto-Indo-European root *ǵneh₃- meaning 'to know.' This PIE root is one of the most widely attested and productive in the entire Indo-European family, with reflexes in virtually every branch and an extraordinary array of English derivatives through both Germanic and Latin channels.
The PIE root *ǵneh₃- produced cognates across the language family with remarkable semantic consistency. Latin 'gnōscere' (later 'nōscere,' to know, to become acquainted with) gave rise to 'cognōscere' (to recognize, to investigate), the source of English 'cognition,' 'recognize,' 'reconnaissance,' 'connoisseur,' and 'cognoscenti.' Latin 'nōbilis' (knowable, famous, noble) comes from the same root, as does 'ignōrāre' (to not know), the source of 'ignore' and 'ignorance.' Greek 'gignṓskein' (to know, to perceive) produced 'gnṓsis' (knowledge), which entered English in 'gnosis,' 'gnostic,' 'agnostic,' 'diagnosis,' and 'prognosis.' Sanskrit 'jñā-' (to know) produced 'jñāna' (knowledge, wisdom), a key term in Hindu and Buddhist philosophy.
Within Germanic, the root *ǵneh₃- was extraordinarily productive. Besides 'know' itself, it produced 'can' (Old English 'cunnan,' to know, to be able — the sense 'to be able' developing from 'to know how'), 'cunning' (originally 'knowing,' from the present participle of 'cunnan'), 'ken' (to know, surviving in Scots English and the expression 'beyond one's ken'), 'keen' (sharp, perceptive — from Old English 'cēne,' bold, wise), and 'couth' (known, familiar — now mainly encountered in its negative form 'uncouth,' literally 'unknown, unfamiliar, strange'). German retains the distinction between 'kennen' (to know a person or thing by acquaintance) and 'wissen' (to know a fact), while English has collapsed both into the single verb 'know.'
Old English Period
The initial consonant cluster /kn-/ in 'know' is one of the most visible fossils of older English pronunciation. In Old English, 'cnāwan' was pronounced with a clearly audible /k/ before the /n/. This cluster was preserved through the entire Middle English period — Chaucer's contemporaries said /knoʊ/, not /noʊ/. The /k/ was dropped only in the seventeenth century, part of a general simplification of initial consonant clusters that also silenced the /k/ in 'knife,' 'knee,' 'knight,' 'knot,' 'knock,' and 'knit.' The spelling preserves the memory of the older pronunciation.
The past tense 'knew' (from Middle English 'knewe,' from Old English 'cnēow') shows the strong verb ablaut pattern characteristic of this verb class. The past participle 'known' continues Old English 'cnāwen.' Both forms have resisted regularization, as expected for a verb of such high frequency.
The noun 'knowledge' has a complex derivation. It comes from Middle English 'knowleche,' formed from 'know' + the suffix '-leche' (of uncertain origin, possibly related to Old English 'lāc,' play, action, or to the '-lock' in 'wedlock'). The word is thus not simply 'know' + '-ledge' but has a more obscure morphological history than its transparent modern spelling suggests.
Modern Usage
Semantically, 'know' in Modern English covers an enormous range: factual knowledge ('I know the answer'), acquaintance ('I know her'), skill ('she knows how to swim'), recognition ('I'd know that voice anywhere'), and experiential understanding ('he has known poverty'). Many languages divide these senses among different verbs — French distinguishes 'savoir' (to know a fact) from 'connaître' (to know a person), mirroring the German 'wissen/kennen' split. English speakers must rely on context to disambiguate.
The philosophical weight of 'know' is immense. Epistemology — the branch of philosophy concerned with knowledge — derives from Greek 'epistēmē' (knowledge), but the English-language philosophical tradition has centered on the verb 'know' and the noun 'knowledge' as its primary terms of art. The classic definition of knowledge as 'justified true belief,' debated since Plato's Theaetetus, remains anchored in the everyday English verb that traces its roots back to the prehistoric *ǵneh₃-.