The Spanish word 'demonio' (meaning 'demon') traces back to Ancient Greek through Late Latin. It began with the Ancient Greek word 'daimon' (δαίμων), which originally referred to a divine power or deity. This evolved into 'daimonion' (δαιμόνιον) in Ancient Greek, meaning a divine being or spirit. When this word was borrowed into Late Latin as 'daemonium', its meaning shifted to specifically refer to an evil spirit or demon. Finally, it evolved into the Spanish 'demonio', maintaining the meaning of 'demon'.

It's interesting to note how the meaning evolved from a neutral or even positive concept of divine power in Ancient Greek to the negative connotation of an evil spirit that we see in Spanish today. This semantic shift occurred primarily during the Christian era, when many pagan religious concepts were reinterpreted in a negative light.

The word 'demonio' has given rise to several related Spanish words that Spanish learners might recognize. These include the adjective 'demoníaco' (demonic), and 'endemoniado' (possessed by a demon). The prefix 'demo-' in these words clearly connects them to their shared origin.

English speakers can easily connect 'demonio' to several similar English words that share the same Greek origin. The most obvious is 'demon', but also 'demonic', 'demonology' (the study of demons), and 'demonize'. A more surprising connection is to the word 'daemon', which in computing refers to a background process, hearkening back to the original Greek meaning of a spirit that works in the background.