The Spanish word 'lo' comes from the Latin word 'illud', which meant 'that' when referring to neuter (non-masculine, non-feminine) things. Over time, as Latin evolved into Spanish, 'illud' was simplified to 'lo' and its meaning broadened to serve as both a neuter article ('the' for abstract concepts) and a neuter pronoun ('it').

In modern Spanish, 'lo' is used in several ways: as a neuter article before adjectives to talk about abstract qualities (like 'lo bueno' meaning 'the good thing/what is good'), and as a direct object pronoun for neuter concepts (like 'lo sé' meaning 'I know it').

The Spanish demonstrative pronouns 'ello' (meaning 'it') and 'él' (meaning 'he/him') are distant relatives of 'lo', as they also evolved from Latin demonstrative pronouns. While 'lo' came from the neuter form 'illud', 'él' came from the masculine form 'ille'.

While there aren't any direct English cognates of 'lo' from the Latin 'illud', English speakers might find it helpful to think of 'lo' as similar to how we use 'it' or 'that' when referring to abstract concepts or ideas. For example, just as we say 'that is good' in English, Spanish uses 'lo' in 'lo bueno' to mean 'what is good' or 'the good thing'.