The Spanish word 'enfermo' (meaning 'sick') comes from the Latin word 'infirmus', which meant 'weak' or 'sick'. The Latin word itself is a combination of two parts: the negative prefix 'in-' (meaning 'not') and 'firmus' (meaning 'firm' or 'strong'). So literally, 'infirmus' meant 'not strong' or 'not firm', which evolved to mean 'weak' and then 'sick'. When this word entered Spanish, the Latin 'in-' prefix evolved into 'en-', giving us the modern Spanish word 'enfermo'.

A related Spanish word is 'firme', which comes directly from Latin 'firmus' and means 'firm' or 'steady'. You can see how 'enfermo' is essentially the opposite of 'firme' - when you're sick, you're not firm or strong. Another related word is 'firmeza' meaning 'firmness' or 'stability'.

Several English words share the same Latin roots as 'enfermo'. The word 'infirm' is a direct borrowing from Latin 'infirmus' and has the same meaning of 'weak' or 'sickly'. We also have 'firm' from Latin 'firmus', and 'firmament' (meaning the heavens or sky, viewed as a solid dome). The word 'infirmary', meaning a place for sick people, is also related. These all share the core idea of firmness or its absence.