The Spanish word 'ardor' comes from the Latin word 'ardor', meaning 'burning' or 'flame'. This Latin noun was formed from the verb 'ardere' meaning 'to burn' combined with the suffix '-or' which was used to create nouns indicating a result or effect. So literally, 'ardor' meant the result or effect of burning.

Some related Spanish words include 'arder' (to burn), 'ardiente' (burning, ardent), and 'arduo' (arduous, difficult). These all share the same Latin root 'ardere' and maintain connections to the concept of burning or intensity.

In English, we have several related words that come from the same Latin source: 'ardent' (passionate, intense), 'arduous' (difficult, requiring great effort), and 'arson' (the criminal act of burning property). The connection between these words lies in the original meaning of burning or heat, which evolved metaphorically to describe intensity of feeling or effort.