'ardor' originates from Latin, specifically the word 'ardor' and the verb 'ardēre', where the root 'ard-' meant 'to burn' or 'to be on fire'.

'ardor' passed into English via Old French 'ardeur' and Middle English 'ardour', and eventually became the modern English word 'ardor' (with the variant spelling 'ardour' in British English).

Initially it meant 'a burning, heat' or physical burning; over time the sense broadened and shifted metaphorically to 'intense feeling or passion', which is the primary modern meaning.