/huː/
[pronoun] Used to ask about or refer to a person or people. It can introduce a question about someone's identity or connect a describing phrase to a person already mentioned.
'Who' descends from a Proto-Indo-European root so ancient it also gave Sanskrit its word for 'who' — the same question asked for over 6,000 years.
The word 'who' is one of the oldest and most unchanged words in the English language. It comes directly from Old English 'hwā', which was used in exactly the same way — to ask about a person's identity. Old English 'hwā' descended from Proto-Germanic '*hwaz', and before that from the Proto-Indo-European root '*kʷo-' or '*kʷe-', a family of interrogative and relative words that formed the backbone of questioning in ancient languages spoken thousands of years before writing existed.
This Proto-Indo-European root is astonishingly productive. It spread across dozens of descendant languages, producing Latin 'quis' and 'qui' (meaning 'who'), Ancient Greek 'tís' (τίς, meaning 'who' or 'which'), and Sanskrit 'kaḥ' (कः, also meaning 'who'). The fact that speakers of Old English, Latin, Greek, and Sanskrit were all asking 'who?' using sounds that trace back to the same prehistoric syllable is one of the great detective stories of historical linguistics.
In Old English, the word had a full set of grammatical forms, much like modern German still does. 'Hwā' was the nominative form (the subject of a sentence), 'hwæm' was the dative (used for indirect objects, which survives today as 'whom'), and 'hwæs' was the genitive form showing possession — which survives as 'whose'. Over centuries, as English lost most of its case endings, this family of forms collapsed, but 'who', 'whom', and 'whose' held on, making them rare survivors of the Old English grammatical system.
The spelling shifted gradually over time. The 'hw-' beginning of Old English was pronounced with an audible puff of breath before the vowel — something like 'hwoo'. During the Middle English period, roughly the 11th to 15th centuries, this 'hw-' spelling was reversed to 'wh-' under the influence of Norman French scribes who found it more natural to write vowel sounds after consonants. Words like 'hwæt' became 'what', 'hwær' became 'where', and 'hwā' became 'who'. The spelling 'wh' was kept, but in most English dialects the 'h' sound quietly disappeared, leaving the simple 'hoo' sound we use today.
Interestingly, the Latin branch of the same root gave English a whole separate family of words through French. Latin 'qui' and 'quod' → Old French 'que' → English 'query', and Latin 'qualis' → Old French 'quel' → English 'quality', and Latin 'quantum' stayed almost unchanged into English 'quantity'. These words feel completely different from 'who', yet they all trace back to that same ancient questioning syllable. 'Who' is the homegrown, Anglo-Saxon version, while 'query' and 'quality' are the Latinate cousins that arrived through the back door.