The Spanish word 'demoniaco' (meaning 'demonic') comes from the Late Latin word 'daemoniacus', meaning 'possessed by a demon'. This Latin term was itself borrowed from Byzantine Greek 'daimoniakos' (δαιμονιακός), also meaning 'demonic'. The word maintained its meaning related to demons and evil spirits as it evolved from Greek through Latin into Spanish, with mainly just phonetic changes occurring along the way.

This word is related to the more common Spanish word 'demonio' (demon), which you might encounter more frequently. Both share the same Greek-Latin root referring to demons or evil spirits. While 'demonio' is the noun for 'demon', 'demoniaco' is the adjective form meaning 'demonic' or 'relating to demons'.

English speakers might recognize the connection to words like 'demon', 'demonic', and 'demoniac'. These English words share the same Greek-Latin ancestry as the Spanish 'demoniaco'. The English word 'demoniac' is particularly close in both meaning and form to the Spanish 'demoniaco', as both come from the Late Latin 'daemoniacus'. While 'demoniac' is somewhat rare in modern English, the related word 'demonic' is very common and carries the same meaning as Spanish 'demoniaco'.