The Spanish word 'quien' (meaning 'who' or 'whom') comes from the Latin word 'quem', which was the accusative form of 'qui'. In Latin, 'qui' was the nominative form meaning 'who' or 'which', while 'quem' was used as the accusative form. As Latin evolved into Spanish, the accusative form 'quem' underwent phonetic changes, with the final 'm' being replaced by 'n', resulting in the modern Spanish 'quien'. This is a common pattern in the development from Latin to Spanish, where final 'm' often became 'n'.

Some related Spanish words include 'que' (that, which) and 'cual' (which), which also derive from Latin interrogative and relative pronouns. These words form part of the Spanish interrogative and relative pronoun system, all sharing ancient roots in Latin question words.

While English 'who' is not directly related to 'quien' (as English is a Germanic rather than Romance language), English has borrowed many words from Latin that are related to 'qui/quem'. These include 'query' (to ask questions), 'inquisitive' (eager to ask questions), and 'quest' (a search or pursuit). All these words share the ancient Indo-European root related to questioning and seeking information.