The Spanish word 'algo' meaning 'something' comes from Latin 'aliquod', which was the neuter accusative form of 'aliquis' meaning 'someone, something'. The Latin word 'aliquis' itself was formed by combining two elements: 'al-' meaning 'other' and 'quis' meaning 'who, what'. Over time, the Latin 'aliquod' evolved and simplified to become the modern Spanish 'algo', while maintaining its basic meaning of 'something'.

This etymology shows how Spanish preserved the indefinite meaning from Latin while simplifying the word's form, dropping the middle syllable 'qu' and the final 'd' to create a shorter, more streamlined word.

A related Spanish word is 'alguien' (meaning 'someone'), which shares the same Latin root 'aliquis'. While 'algo' refers to things ('something'), 'alguien' refers to people ('someone'), showing how Spanish developed distinct words for indefinite references to objects versus persons.

While English 'something' and 'someone' are not etymologically related to Spanish 'algo', the interrogative element 'quis' in its Latin origin is related to English question words like 'who' and 'what'. These all ultimately come from Proto-Indo-European interrogative roots, showing a distant connection between Spanish 'algo' and English question words.