The Spanish word 'alguien' meaning 'someone' comes from the Latin word 'aliquis' (specifically its accusative form 'aliquem'), which also meant 'someone' or 'anybody'. The word underwent phonetic changes as Latin evolved into Spanish, with 'qu' becoming 'gu' and the final '-em' developing into '-en', giving us the modern Spanish form 'alguien'.

A related Spanish word is 'algo' meaning 'something', which comes from the same Latin root 'aliquid' (the neuter form of 'aliquis'). Both 'alguien' and 'algo' share the idea of referring to an unspecified person or thing.

While there aren't any common English words directly related to 'alguien', English speakers might find it helpful to remember that 'alien' (though etymologically unrelated) has a similar initial sound and also refers to an unspecified 'other' person, albeit with a different specific meaning.