The Spanish word 'bastardo' meaning 'illegitimate child' comes from Old French 'bastard' with the same meaning. The Old French term itself has interesting origins, tracing back to the Proto-Germanic word 'banstiz' meaning 'barn' or 'granary'. This semantic evolution likely occurred because illegitimate children were often conceived in outbuildings like barns, leading to the term eventually becoming associated with children born out of wedlock.

There aren't many common Spanish words that are directly related to 'bastardo', as it stands somewhat isolated in modern Spanish vocabulary. The word is considered rather harsh or vulgar in modern usage, and more polite terms are often preferred.

The English word 'bastard' is directly related to 'bastardo', as they both come from the same Old French source. While it originally had the same meaning of 'illegitimate child', in modern English it has also taken on additional meanings as a general insult or to describe something that is of irregular, inferior, or dubious origin. Like its Spanish cousin, it's considered a vulgar or offensive term in most contexts.