The Spanish word 'bruja' (meaning 'witch') has an interesting Celtic origin. It comes from the Proto-Celtic word 'brixta', which meant 'magic spell'. This word evolved into 'bruxa' in Medieval Spanish, maintaining its connection to the supernatural but specifically referring to a 'witch'. Finally, it developed into the modern Spanish form 'bruja', which is still used today to mean 'witch'.

This etymology shows how the meaning shifted slightly from the concept of a magical spell itself to the person who casts such spells. It's also interesting to note that this Celtic influence on Spanish vocabulary reflects the historical presence of Celtic peoples in the Iberian Peninsula before the Roman conquest.

The word 'bruja' is quite unique in Spanish and doesn't have many common related words in modern Spanish. However, you might encounter related terms like 'brujería' (witchcraft) and 'embrujar' (to bewitch), which are derived directly from 'bruja'.

While there aren't direct English cognates of 'bruja' (as English witch-related vocabulary tends to come from Germanic roots rather than Celtic ones), English has borrowed some related terms through popular culture, such as 'brujo/bruja' sometimes appearing in English texts about Latin American mysticism and witchcraft.